/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isSymmetricVector(const std::vector<const TreeNode*>& nodes) {
        if (nodes.empty())     return true;
        if (nodes.size() == 1) return true;
        for (size_t i = 0; i < nodes.size()/2; ++i) {
            const size_t j = nodes.size() - i - 1;
            if ((nodes[i]  && !nodes[j]) ||
                (!nodes[i] && nodes[j]) ||
                (nodes[i]  && nodes[j] && (nodes[i]->val != nodes[j]->val))) {
                return false;
            }
        }
        return true;
    }

    bool isSymmetricBfs(std::vector<const TreeNode*>& current_level,
                        std::vector<const TreeNode*>& next_level) {
        if (current_level.empty()) {
            return true;
        }
        if (!isSymmetricVector(current_level)) {
            return false;
        }
        for (const TreeNode* node : current_level) {
            if (node) {
                next_level.push_back(node->left);
                next_level.push_back(node->right);
            }
        }
        current_level.clear(); // New next level.
        return isSymmetricBfs(next_level, current_level);
    }

    bool isSymmetric(TreeNode* root) {
        if (!root) return true;
        std::vector<const TreeNode*> current_level({root});
        std::vector<const TreeNode*> next_level;
        return isSymmetricBfs(current_level, next_level);
    }
};