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.

用DFS搞定

1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     int maxDepth(TreeNode *root) {13         // Start typing your C/C++ solution below14         // DO NOT write int main() function15         if (root == NULL)16             return 0;17             18         int leftDepth = maxDepth(root->left);19         int rightDepth = maxDepth(root->right);20         21         return max(leftDepth, rightDepth) + 1;22     }23 };