/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        if(root==NULL){
            return true;
        }
        return isSys(root->left,root->right);
    }
    bool isSys(TreeNode *left,TreeNode *right){  // 判断父节点是否相等 在判读左子树和右子树
        if(left==NULL&&right==NULL){
            return true;
        }else if(left==NULL||right==NULL){
            return false;
        }
        if(left->val!=right->val){
            return false;
        }
        return isSys(left->left,right->right)&&isSys(left->right,right->left); //
    }
};