An old favorite, which I got at a Google interview. Not understanding the Google mindset at the time (2007 or so), I had my engineer hat on first, and pointed out that if you’re going to build a data structure, build the damn data structure. It’s ludicrous to build two stacks when you could build one queue in approximately the same amount of time.
However, the real purpose was “do you understand that pushing onto one stack and then popping items off onto the other reverses their order?”. Sure. OK. Fine. You could ask me “what would happen if…”, but no, it needs to be a no-one-with-any-brains-would-ever-do-this programming problem. (Again, this is a “let’s talk about your algorithm choices in our next 1-1” problem.)
Anyway, yeah. So I tried with arrays again to start and it was a huge PITA. I tried that for about ten minutes and said, forget this, I’m building it with linked lists.
So we have a Node that is added at the head to push, and delinked to its tail to pop. Easy-peasy.
The push operation just adds to the incoming stack, and doesn’t do anything else until we have our first pop, and which point we call the core routine here, Cycle. Cycle pops items one at a time off the in stack and pushes them to the out stack. Now a pop pulls the first item that was pushed, as required, since we’ve just reversed the in stack. I did have one error, caught in the submit, in that you don’t just check for the in stack being empty, but you also need to check for the out stack not being empty. If you don’t, you end up queueing an item out of order. Always wait for the output to be empty before actually cycling.
I did have to refresh my memory on adding a method to a struct – it’s func (this *Node) Cycle – but otherwise this was almost as much a breeze as the tree inversion.
I do need to do some exercises on managing arrays better, though — you can’t always use a linked list, especially if you need actual indexing.
Overall, pleased with the solution: 82nd percentile runtime, but 18th percentile memory. Might be better if I kept the popped nodes on a free list.
type Node struct {
Val int
Next *Node
}
type MyQueue struct {
In *Node
Out *Node
free *Node
}
func Constructor() MyQueue {
return MyQueue{
In: nil,
Out: nil,
}
}
func(this *MyQueue) Cycle() {
fmt.Println(">>>cycling in to out")
if this.In == nil {
// Nothing to cycle
fmt.Println(">>cycle empty in")
return
}
if this.Out != nil {
// Don't cycle yet
return
}
// Pop items off the in stack, push on the out stack
for {
current := this.In
this.In = this.In.Next // Pop In
fmt.Println("cycle>> pop in", current.Val)
current.Next = this.Out // Push Out
this.Out = current
current = this.In
if current == nil {
break
}
}
// Clear in stack
fmt.Println("cycle>> clear in")
this.In = nil
}
func (this *MyQueue) Push(x int) {
// Append to in stack. Leave there until we cycle.
n := &Node{
Val: x,
Next: this.In,
}
this.In = n
fmt.Println(">>push on in", x)
}
func (this *MyQueue) Pop() int {
if this.Empty() {
panic("popped empty stack")
}
if this.Out == nil {
this.Cycle()
}
value := this.Peek()
fmt.Println(">>pop Out")
this.Out = this.Out.Next
return value
}
func (this *MyQueue) Peek() int {
if this.Empty() {
panic("peek on empty stack")
}
this.Cycle()
fmt.Println("peek>>")
return this.Out.Val
}
func (this *MyQueue) Empty() bool {
fmt.Println("check for empty")
return this.In == nil && this.Out == nil
}
/**
* Your MyQueue object will be instantiated and called as such:
* obj := Constructor();
* obj.Push(x);
* param_2 := obj.Pop();
* param_3 := obj.Peek();
* param_4 := obj.Empty();
*/
Editorial note: other solutions submitted really stretch the definition of stack – a lot of them do do appends and slices, but of a single array, which is kind of not implementing two stacks at all.
Note 2: adding a free queue of nodes improves memory usage by 1%. Not worth it.
I started out with the simplest possible thing that could work: when looking to see if you have more than N of a thing, use a map.
**
* Definition for singly-linked list.
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
func hasCycle(head *ListNode) bool {
seen := make(map[*ListNode]bool)
for scan := head; scan != nil; scan = scan.Next{
_, ok := seen[scan]
if ok {
return true
}
seen[scan] = true
}
return false
}
Small, simple, but not very fast. 14th percentile in speed, 8th percentile in memory.
I vaguely remembered that there was a pointer-chasing way to do this, and that’s the fast and small way. (This falls into the “do you know the trick” category, which I kinda hate.)
func hasCycle(head *ListNode) bool {
slow := head
fast := slow
if head == nil {
return false
}
for {
// if we'd run off the end, stop.
if slow.Next == nil || fast.Next == nil || fast.Next.Next == nil {
return false
}
slow = slow.Next
fast = fast.Next.Next
if fast == slow {
return true
}
}
return false
}
Considerably faster: 95th percentile in speed, 45th in memory, which is interesting.
Now we start getting into “I would fire you if you did this” solutions.
How about if we just overwrite the value with an impossible one, and if we see the impossible value, return true (because it couldn’t be there unless we put it there)? It’s slower, which is to be expected, since we’re modifying the nodes and looking at them, not just looking at pointers. But surprisingly, the memory usage is still around 43%. And the list contents are trashed.
/**
* Definition for singly-linked list.
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
const invalidValue = 99999999
func hasCycle(head *ListNode) bool {
// Destructive version.
scan := head
if head == nil {
return false
}
for {
// if we'd run off the end, stop.
if scan.Next == nil {
return false
}
if scan.Val == invalidValue {
return true
}
scan.Val = invalidValue
scan = scan.Next
}
return false
}
So what gets us better memory usage? How about breaking the links, and pointing them to na-na land, and stopping if we get a pointer to na-na land?
/**
* Definition for singly-linked list.
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
func hasCycle(head *ListNode) bool {
// Another destructive version.
scan := head
// Create an off-in-nowhere node.
tumbolia := &ListNode{9999999, nil}
if head == nil {
return false
}
for {
if scan == tumbolia {
// we're in na-na land
return true
}
// if we'd run off the end, stop.
if scan.Next == nil {
return false
}
// point the next of this node to na-na land.
here := scan
scan = scan.Next
here.Next = tumbolia
}
return false
}
No major improvement. 90th percentile in speed, 43rd percentile in memory. List structure is trashed.
So overall, the Floyd two-pointer is the winner; would love to see something that uses less memory, but haven’t found anything yet.
I had a really hard time getting my head around this problem, which contributed to major flailing on my part trying to get it sorted. I had several almost-right solutions, but I kept not getting the recursion right. I eventually sorted it out to “I need to check the subtree heights” as the right attack, but didn’t finally sort it until the 10th try or so.
This wold absolutely have been a fail in an interview. I’m getting much better with pointers – I had no trouble getting the recursions to work, and never had a nil pointer problem – but I just could not get the concept clear enough in my head, and that lead to my just not getting it right in the code.
After a lot of flailing I finally came up with a height-checking algorithm that was the best shot at it I could do, but I had to finally check with the solutions to spot my error, which was treating the left and right subtrees separately when they needed to be unified.
Final version after the fix did okay in the rankings: top 84% in speed and top 81% in memory, but the path to getting it was really bad. I need to spend a lot more time on tree-crawling.
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func isBalanced(root *TreeNode) bool {
if root == nil || (root.Left == nil && root.Right == nil) {
return true
}
_, which := heightCheck(root)
return which
}
func heightCheck(root *TreeNode) (int, bool) {
if root == nil {
return 0, true
}
lH, lB := heightCheck(root.Left)
rH, rB := heightCheck(root.Right)
// I was way overcomplicatng this, tracking the
// branches separately and not sending along a
// give-up flag. Doing that helped a lot.
if !lB || !rB || abs(lH - rH) > 1 {
return -1, false
}
return max(lH, rH) + 1, true
}
func abs(a int) int {
if a > 0 {
return a
}
return -a
}
func maxDepth(root *TreeNode) int {
if root == nil {
return 0
}
left := maxDepth(root.Left)
right := maxDepth(root.Right)
return max(left, right)+1
}
func max(a,b int) int {
if a>b {
return a
}
return b
}
Interestingly, if one adds a second recursion shortcut check for leaves, the memory usage gets much better: 99th percentile, but the execution time gets much worse: 54th percentile.
First, shout-out to ShineX Media for setting me on the right track for this.
If you use modern versions of Photoshop, you’ve probably noticed that in the “New file…” dialog, there’s a “Recent” tab, which is the default one that comes up. If you’ve been using it to create files in different shapes and sizes and formats, you’ll notice that it gets pretty cluttered up, and it’s easy to accidentally, say, pick something with the right dimensions but wrong DPI and then waste a chunk of time working on an unusable document.
Adobe has provided no built-in way to clear this. People have been asking since 2016 for a solution.
The one documented on the Adobe site, go to the preferences and turn this version of the dialog off, is like the old joke:
“Doctor, my shoulder hurts when I raise my hand above waist level.”
“Well…don’t do that.”
I mean, it’s a solution; it’s just not a very reasonable one.
I spent a fair amount of time googling around and finally found a reference to Shine X Media’s video that explains how to do it for Windows; essentially you close Photoshop (very important to keep PS from just rewriting the file again), go looking through the AppData directory for a file named MRU New Doc Sizes (MRU is Most Recently Used), and remove it.
On the Mac, this lives in "${HOME}/Library/Preferences/Adobe Photoshop 2024 Settings/MRU New Doc Sizes.json", and you can simply use the Go… option in the Finder to navigate to the Adobe Photoshop 2024 Settings directory and drag it to the trash.
Again, make sure Photoshop is not running!
This is confirmed to work fine on my 2024 install under Sonoma. Happy and clean open dialogs to you!
This is a tougher one, and honestly, you would have needed to have solved this yourself before to have gotten to a reasonable algorithm in the usual 1/2 hour time. It’s generally a significant sign if the problem description has to refer to Wikipedia for an accurate description of the problem.
This is one that is very simple to do by inspection, but difficult to do simply by algorithm, let alone an efficient one.
My first idea was that I’d need to know the paths to the nodes, and that I’d *handwave* figure out the common ancestor by using those. The good algorithm for this kind of uses the idea of paths to the nodes, but instead it does an in-order trace of the paths through the tree – an Euler tour, dude comes in everywhere – and that allows us to transform the problem into a range-minimum query (e.g., when I have node P and node q, and the path I needed to get from one to the other, at some point in there I’ll have a minimum-depth node, which will either be one of the two, or a third higher node).
The Wikipedia example shows this as an illustration:
To spell this out more explicitly: you can see we did the in-order tour, transforming the tree into a linear walk through it: A, down to B, down to C, back up to B, down to D, down to E, up to D, and so on. We record the node value and the depth at each step. To get the range query, we want the first appearance of each of two nodes we’re searching for (and we can record that as we’re crawling), and then to look for the highest node in the range of nodes including both of them. For the example in the diagram, step 6 is where we find the first node, E, and step 9 is where we find the second node, F. Iterating from step 6 to step 9, the node with the lowest depth in that range is B, and we can see from inspecting the tree that this is correct.
So.
At this point I’ve explained the solution, but have not written any code for it at all yet. Better go get that done.
Note: I’ve gotten the Euler crawl to mostly work, but I’m having trouble with the returned data from the crawl. Debugging…and yes, I am too cheap to pay for leetcode’s debugger. fmt.Println FTW.
The crawler crawls, but the output is wrong. I’ve tracked this down to a fundamental error on my part: Go arrays are not built for this kind of thing. I am switching the crawler over to building a linked list, which I think I only have to traverse one direction anyway, so it’s just as good.
This definitely was a failure to time-block. I will keep plugging on this until I actually finish solving it, but this would have been a DQ in an interview.
I’ve now discarded two different Euler crawls, and have a working one…but the range-minimum isn’t working as I expect it to. Will have to come back to it again. I begin to wonder if trying to track the start node is, in fact, a bad idea, and if I should simply convert the linked list I’m generating in the crawl to an array for the range minimum once it’s returned.
The crawl now returns the head and tail of a list that contains the visit order for the Euler crawl: node, left subtree in order, node again, right subtree in order, node again. This breaks down like this:
If the current head is nil, return [nil, nil].
Generate a crawl node for just this node at the current level
Set head and tail to this node
If the current head points to a leaf node:
Return head and tail.
Otherwise:
Crawl the left side at level+1, getting a leftside head and tail.
Crawl the right side at level+1, getting a rightside head and tail.
If the leftside crawl is non-null:
Set tail.next to leftside head.
Create a new crawl node for the current node at the current level.
Set leftside tail node's next to this new node.
Set tail to this middle node.
If the rightside crawl is non-null:
Set tail.next to the rightside head.
Create a new crawl node for the current node at the current level.
Set rightside tail node's next to this new node.
Set tail to this new node.
Return head and tail.
This returns a singly-linked list with an in-order traversal, recording every node visited and the tree level at which it was seen. We should now be able to find the start and end nodes, and scan the list between the two inclusive, looking for the highest node, and that should find the LCA.
The scan is a bit of a bastard as well. The nodes may appear in any order, and we have to find the start and end no matter which order they show up in. I initially tried swapping them if they were out of order, but this didn’t work as I expected – that may have been other bugs, but I coded around it in the final scan code.
The final scan first says “I don’t have either node” and skips nodes until it finds one of them. Once it does, it starts looking for the node with the highest level. The tricky bit is that we need to detect that we’ve found the first node, then start checking, and only stop checking after we find the end node (as it may be the highest, since nodes are considered their own roots).
Here’s the final code. It did not do well in the rankings: bottom 5% in speed, and bottom 25% in memory usage. I suspect I could improve it by merging the “find the local high node” code into the Euler crawl, and by not actually recording the nodes in the linked list at all but passing the state around in the recursion.
Lessons learned:
Do not try to do complicated array machinations in Go. Better to record the data in a linked list if you’re doing something that needs to recursively append sublists.
Think more clearly about recursion. What is the null case? What is the one-element case? What is the more-than-once case? What is the termination criterion for the more-than-one case?
Don’t be afraid to discard an old try and do something different. Comment out the failed code (and note to the interviewer that you’d check the broken one in on a local branch in case you wanted to go back to it).
Continuing to practice with Go pointer code is necessary. I’m a bit weak with pointers.
This took way too long to do, and way too many shots at getting to something that would work at all. I grade myself a fail on this one. I might come back at a later date and try to make it faster and smaller.
Addendum: removed the debugging and the dump of the crawl output. Even without doing anything else, that bumped my stats to the top 73% in runtime and top 99% in memory usage, and that is a pass with flying colors!
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
type eulerNode struct {
node *TreeNode
depth int
next *eulerNode
}
func lowestCommonAncestor(root, p, q *TreeNode) *TreeNode {
// Use the transformation of this problem into a range-minimum one.
// Crawl the tree in-order, creating an Euler path. Each step records
// the node value and the depth.
head, tail := eulerCrawl2(root, p, q, 0)
// dump crawl output - note that removing this should make the code
// significantly faster
scan := head
for {
if scan == nil {
break
}
fmt.Println(scan.node.Val, "depth", scan.depth)
scan = scan.next
}
fmt.Println("crawl complete", head, tail)
// Start is the beginning of the range, and the end of the path is the end.
// Find the path minimum in the range (the lowest depth value).
var ancestor *TreeNode
minDepth := 9999999999
fmt.Println("Start at", head.node.Val, head.depth)
foundP := false
foundQ := false
for scan := head; scan != nil; scan = scan.next {
fmt.Println("node", scan.node.Val, scan.depth)
if scan.node.Val == p.Val {
fmt.Println("found p")
foundP = true
}
if scan.node.Val == q.Val {
fmt.Println("found q")
foundQ = true
}
if !foundP && !foundQ {
fmt.Println("skip", scan.node.Val)
continue
}
fmt.Println("check", scan.node.Val, scan.depth)
if scan.depth < minDepth {
// New range minimum
minDepth = scan.depth
ancestor = scan.node
fmt.Println("new candidate min at", scan.node.Val, scan.depth)
}
if foundP && foundQ {
fmt.Println("found and processed complete range")
break
}
}
fmt.Println("ancestor is", ancestor.Val)
return ancestor
}
func eulerCrawl2(root, p, q *TreeNode, depth int) (*eulerNode, *eulerNode) {
// Nil tree returns nil pointer
if root == nil {
fmt.Println("skipping nil pointer")
return nil, nil
}
// Non-nil next node. Generate a crawl node for this tree node.
fmt.Println("adding crawl node for", root.Val)
head := &eulerNode{
node: root,
next: nil,
depth: depth,
}
tail := head
// If this is a leaf, the crawl of
// this node is just this node. Head and tal point to the same node.
if root.Left == nil && root.Right == nil {
fmt.Println("returning at leaf")
return head, tail
}
// Non-leaf. Crawl will be this node, the left subtree at depth+1,
// another copy of this node at this depth, the right subtree at depth+1,
// and one more copy of this node.
fmt.Println("crawling down")
leftSubtreeHead, leftSubtreeTail :=
eulerCrawl2(root.Left, p, q, depth+1)
rightSubtreeHead, rightSubtreeTail :=
eulerCrawl2(root.Right, p, q, depth+1)
// Chain the results together:
// - this tree node
// - the left crawl below it
// - another copy of this node
// - the right crawl below it
// - a final copy of this node
// Link this node to the start of the list from the left crawl
if leftSubtreeHead != nil {
// If there is a crawl below, link it in, and add another
// copy of the current node for coming back up
tail.next = leftSubtreeHead
leftSubtreeTail.next = &eulerNode{
node: root,
next: nil,
depth: depth,
}
tail = leftSubtreeTail.next
}
if rightSubtreeHead != nil {
// right crawl exists. Add it at the current tail, reset tail.
tail.next = rightSubtreeHead
rightSubtreeTail.next = &eulerNode{
node: root,
next: nil,
depth: depth,
}
// reset tail to new last node
tail = rightSubtreeTail.next
}
// Return head and tail of the newly-generated list of crawled
// nodes.
fmt.Println("returning results at depth", depth)
return head, // Head of this new list
tail // Tail of this new list
}
A bit of a break working on a friend’s art business launch; still working on that, but made some time to come back to leetcode month today.
Today’s problem is flood fill; another easy-rated one. This is again a recursion problem, and the tricky bit is as usual preventing extra recursion that doesn’t need to happen.
Doing this one is made a lot easier by unifying the calling sequence with a helper function that adds an
Go-specific issues for me were properly extracting the dimensions of a two-D array and having to do that with a pointer. Since we want to alter the array we’re flood filling, we need to pass a pointer to it so that we can actually modify the source array.
I got tripped up by the termination condition. In addition to “are we outside the bounds of the array”, we also need to check if this cell needs filling at all (is it the fill color already? Is it the color we want to fill?).
I did need a couple runs with debug prints in to spot where my recursion was going wrong; the first cut didn’t have the “is this already the right color” test, and I was able to catch that one with the supplied test cases. It. failed on the submit because I wasn’t checking for “is the fill already done” case.
How did I do relative to other cases? Pretty good! Runtime in the top 92%, memory usage a quarter of a percent short of the 100th percentile. Should be a pass in an interview. (I did get this one at Thumbtack, and came out with a solution similar to this one, except I used an 8-cell neighborhood when a 4-cell will work. Too many Game-of-Life programs.)
Should have been an easy one, but I haven’t written one by hand in years. Ended up Googling.
Some interesting notes on the code:
for with a conditional break inside is considerably slower than for condition.
The version of this that I learned back when used a step division, and that actually is much harder to get right. The upper/lower/mid version is much easier to get right.
I decided to not do a recursive solution this time on purpose. The expressivity that recursion gives you is not to be denied.
Final version scored well: 90% on speed, 97% on memory. Acceptable, but the amount of dicking around was not. Note: study up more of these common but not commonly coded algorithms.
func search(nums []int, target int) int {
// Find midpoint, check element; equal, less, or greater.
// if the step size goes to 0 we did not find.
low := 0
high := len(nums) -1
for low <= high {
mid := (high + low )/ 2
if nums[mid] < target {
low = mid + 1
} else if nums[mid] > target {
high = mid -1
} else {
return mid
}
}
return -1
}
Problem: two strings. Are they anagrams of each other?
Software engineer hat again: if these are arbitrary Unicode strings, then the right solution is count the characters in the first string, binning the counts by character, and then subtract the counts of the characters in the second string. If the characters are ASCII, then you can be clever and use the character value as an index into an array and do the same add/subtract algorithm…but someone will use a non-ASCII character, sooner or later. And the failure mode of clever is…well.
Score: bottom 15% in time and space, but I’ll take that for the vastly reduced need to worry about weird input. If we remove the conversion to string and use bytes, then it improves to 30%. The real win doesn’t happen until the conversion to arrays. I initially had a delete to remove counts that went to 0 and a check for counts that went negative (chars in t not in s) but there wasn’t a significant speedup.
func isAnagram(s string, t string) bool {
// easiest way is to map the character counts into a hash.
// if the two strings match, there should be no odd counts.
if len(s) != len(t) {
return false
}
charCounts := make(map[byte]int)
for i := range(s) {
charCounts[s[i]]++
}
for i := range(t) {
x := string(t[i])
charCounts[x]--
}
for _, count := range(charCounts) {
if count != 0 {
return false
}
}
return true
}
Giving up and using an array of 256 ints, indexed by the int value of the bytes, it’s much faster: 93% on time, 98% on space. Good enough. I suppose you could use this on Unicode input, since it’s just looking at raw bytes. Depends on whether you have a character that can be represented in more than one way in Unicode.
The syntax for creating arrays continues to be a problem. I’m going to standardize on assigning the type followed by {} to the variable.
I chose the “busy software engineer” approach: I know an algorithm that will work and is trivial to implement. Implement that.
To remind everyone what inverting a tree is: if criterion for insertion is left branch < right, then then output tree should have left branch > right. The dead simple algorithm is top-down: save the left, assign the right to the left, assign the saved to the right, recurse.
It’s clear, it’s simple, and fast to code. I literally had a working solution in 2 runs (forgot the null root case on the first run).
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func invertTree(root *TreeNode) *TreeNode {
if root == nil {
return nil
}
invertTree(root.Left)
invertTree(root.Right)
t := root.Left
root.Left = root.Right
root.Right = t
return root
}
About as simple as you can possibly get. Scorewise, it’s fast: top 77%. Memorywise, it’s pretty poor: bottom 8%.
I’m betting a pointer crawl would use less memory but wouldn’t necessarily be that much faster. Let’s see.
First note, still writing append as if this were Scala. Sigh.
This try ends up in an infinite loop; I’m mismanaging the stack somehow. (See why letting Go do it is better?) I’m declaring this good enough and moving on. I expect I could write a version where I manage the stack myself, but this is why we invented higher-level languages.
Unless our trees are always incredibly deep, this is the right way to do this. Even skipping the recursion for nil subtrees doesn’t really make it faster.
On the other hand, I solved the initial recursive version in about 5 minutes, so I’m getting my feet back under me.
I’m perceiving a pattern in my failures here and it is 100% “did you absolutely understand the requirements”.
Detecting a palindromic string was the easy part: just put an index at the end and the beginning, and march them toward the middle. If they meet or pass each other, then we succeeded. If at any point the characters at the indexes don’t match, we fail.
The problematic part was the cleanup of the initial string. Lowercasing wasn’t bad, though I had to look up where the function lived (it’s strings.ToLower()). Filtering the input was harder. I had to go check Stack Overflow and found this code, which is very nice and will go in my personal toolbox:
The strings.Map function is really nice for “do something for every rune in a string”, especially since the special -1 return value says “drop this character”. Can do just about any transliteration you want with that! The unicode.IsDigit function highlights my “you did not read the instructions” error: I assumed that it would be alpha only, and that was incorrect; it was alphanumeric input.
I did spot a useful optimization: if the string is less than two characters long, by definition it’s a palindrome, so we can skip scanning it altogether.
Otherwise the algorithm I coded to scan was right the first time.
How’d I do comparatively? Meh. Runtime in the bottom 50%, memory in the top 60%. I think the lowercase followed by the cleanup is the culprit, and doing it all in one loop would be faster. I don’t think it’s as clear.
I tried a couple variations, and all of them are worse:
Reverse the string and compare it to itself
Inline the cleanup
This just may be something that Go kind of sucks at doing.
func isPalindrome(s string) bool {
// lowercase, skim out all non-alpha characters
s = strings.ToLower(s)
s = spaceMap(s)
if len(s) < 2 {
return true
}
begin := 0
end := len(s) - 1
for {
if begin > end {
return true
}
if begin == end {
return true
}
if s[begin] != s[end] {
return false
}
begin++
end--
}
}
func spaceMap(str string) string {
return strings.Map(func(r rune) rune {
if !unicode.IsDigit(r) && !unicode.IsLetter(r) {
return -1
}
return r
}, str)
}
