判断一颗树是否是镜像

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isSymmetric(TreeNode root) {        if(root==null)            return true;             return symmetric(root.left,root.right);    }     public boolean symmetric(TreeNode left,TreeNode right){         if(left==null&&right==null)             return true;         else if(left!=null && right==null)             return false;         else if(left==null && right!=null)             return false;         else{             if(left.val==right.val){                 return symmetric(left.left,right.right) && symmetric(left.right,right.left);             }else{                 return false;             }         }     }}