递归：

很简单，就递就完事了

 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int maxDepth(TreeNode* root) {
        // write code here
        if(root == nullptr) return 0;
        int left = maxDepth(root->left)+1;
        int right = maxDepth(root->right)+1;
        return max(left,right);
    }
};