class Solution {
public:
    bool compare(TreeNode* left, TreeNode* right) {
        if(left == nullptr && right == nullptr) return true;
        if(left != nullptr && right == nullptr) return false;
        if(left == nullptr && right != nullptr) return false;
        if(left->val != right->val) return false;

        bool c1 = compare(left->left, right->right);
        bool c2 = compare(left->right, right->left);
        return c1 && c2;
    }

    bool isSymmetric(TreeNode* root) {
        if(root == nullptr) return true;
        return compare(root->left, root->right);
    }
};

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root == nullptr) return true;
        queue<TreeNode*> que;
        que.push(root->left);
        que.push(root->right);

        while(!que.empty()) {
            TreeNode* left = que.front(); que.pop();
            TreeNode* right = que.front(); que.pop();

            if(left == nullptr && right == nullptr) continue;
            if(left != nullptr && right == nullptr) return false;
            if(left == nullptr && right != nullptr) return false;
            if(left->val != right->val) return false;
            que.push(left->left);
            que.push(right->right);
            que.push(left->right);
            que.push(right->left);
        }

        return true;
    }
};