101. Symmetric Tree

/**
 * 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 areSymmetricSubtree(root->left,root->right);

    }
    bool areSymmetricSubtree(TreeNode * left, TreeNode* right) {
        if(!left||!right) return !left&&!right;
        return left->val==right->val &&  areSymmetricSubtree(left->left,right->right) && areSymmetricSubtree(left->right,right->left);

    }
};