/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int maxDepth(TreeNode root) {        if (root == null)            return 0;        int left = maxDepth(root.left);        int right = maxDepth(root.right);        int x = left>right ? left+1:right+1;        return x;    }}