题目描述：

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.

解题思路：直接DFS。

代码：

/**
 * Definition for binary tree
 * 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(root == NULL)
            return 0;
        int leftLength = 0;
        int rightLength = 0;
        if(root->left != NULL)
            leftLength = maxDepth(root->left);
        if(root->right != NULL)
            rightLength = maxDepth(root->right);
        if(rightLength > leftLength)
            return rightLength + 1;
        else
            return leftLength + 1;
    }
};

LeetCode:Maximum Depth of Binary Tree