Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

3   / \  9  20    /  \   15   7

return its depth = 3.

 

time: O(logn), space: O(logn)

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public int maxDepth(TreeNode root) {        if(root == null) {            return 0;        }        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));    }}