/**
* Definition for a binary tree node.
* 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) return true;
return helper(root -> left, root -> right);
}
bool helper(TreeNode* p, TreeNode* q) {
if (p == NULL || q == NULL) return p == NULL && q == NULL;
if (p -> val != q -> val) return false;
return helper(p -> left, q -> right) && helper(p -> right, q -> left);
}
};