LeetCode 104. Maximum Depth of Binary Tree

题目：
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.

思路：
使用递归求解非常容易，一个节点的深度等于它的左子树的深度和它的右子树的深度的最大值加上1，空树的深度是0.

代码：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int maxDepth(TreeNode* root) {       if (NULL == root){         return 0;       }       int l = maxDepth(root->left);       int r = maxDepth(root->right);       return l > r ? l + 1:r+1;     }};