104. Maximum Depth of Binary Tree

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.

难度：easy

题目：给定二叉树，找出最大深度。
最大深度指的是最长路径从根结点到最远的叶子结点。

思路：后序遍历

Runtime: 0 ms, faster than 100.00% of Java online submissions for Maximum Depth of Binary Tree.
Memory Usage: 27.8 MB, less than 73.06% of Java online submissions for Maximum Depth of Binary Tree.

/**
 * 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 (null == root) {
            return 0;
        }
        
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}