/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null){return true;}

        return judge(root.left,root.right);
    }
    boolean judge(TreeNode left,TreeNode right){
        // 左子树和右子树都是空
        if(left == null && right == null){
            return true;
        }
        // 左子树空，右子树非空
        if(left == null && right != null){
            return false;
        }
        // 右子树空，左子树非空
        if(right == null && left != null){
            return false;
        }
        // 左右字子树不相等
        if(left.val != right.val){
            return false;
        }
        // 左右子树值相等，则判断其下层节点
        if(left.val == right.val){
            return judge(left.left,right.right)&&judge(left.right,right.left);
        }
        return true;
    }
}