二叉树的最大深度

简洁写法：

/**
 * 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 dfs(TreeNode* root)
    {
        if(root==NULL) return 0;
        return max(dfs(root->left),dfs(root->right))+1;
    }
    int maxDepth(TreeNode* root) {
        return dfs(root);  
    }
};