class Solution {
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res {};
        if (!root) return res;

        queue<TreeNode*> q {{root}};
        bool flag = true;

        while (!q.empty()) {
            auto size = q.size(); 
            vector<int> vLevel(size);
            for (size_t i = 0; i < size; i++) {
                auto node = q.front(); q.pop();
                auto idx = flag? i : size-1-i;
                vLevel[idx] = node->val;
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            flag = !flag;
            res.emplace_back(move(vLevel));
        }

        return res;
    }
};