7 lines c++ solution
/\nThe key is to traverse the left subtree with order root -> left -> right, \nand the right subtree with order root -> right-> left\n/\n\n\n    bool isSymmetri
/*\nThe key is to traverse the left subtree with order root -> left -> right, \nand the right subtree with order root -> right-> left\n*/\n\n\n    bool isSymmetric(TreeNode* root) {\n        if (!root) return true;\n        return isSymmetric_helper(root->left, root->right);\n    }\n    bool isSymmetric_helper(TreeNode* root1, TreeNode* root2) {\n        if (root1==NULL && root2==NULL) return true;\n        if (root1==NULL || root2==NULL) return false;\n        if (root1->val != root2->val) return false;\n        return isSymmetric_helper(root1->left, root2->right) && isSymmetric_helper(root1->right, root2->left);\n    }
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