DFS:104. Maximum Depth of Binary Tree

最大深度，典型的dfs。差点又想加一个if(root->left == NULL && root->right == NULL)，没必要。代码很简单：

/**
 * 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(root == NULL )
            return 0;
        else
            return max(maxDepth(root->left), maxDepth(root->right)) + 1;
    }
};