After other problems related to heights, this is an easy one. Find the maximum distance from the root to a leaf in a binary tree.

My first cut had an extra check for the null-left, null-right node, but I realized that the extra call with a nil pointer was faster than an extra conditional in the main path.

**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func maxDepth(root *TreeNode) int {
    if root == nil {
        return 0
    }
    return 1 + max(maxDepth(root.Left), maxDepth(root.Right))
}

func max(a, b int) int {
    if a >= b {
        return a
    }
    return b
}

100% on speed, 26% on memory. Finished about as fast as I could type it.