/**
 * 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:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if(root == nullptr) return ans;
        queue<TreeNode*> q;
        q.push(root);
        bool isOrder = true;
        while(q.size()) {
            deque<int> dq;
            int n = q.size();
            for (int i = 0; i < n; i ++) {
                auto node = q.front();
                q.pop();
                if(isOrder) dq.push_back(node->val);
                else dq.push_front(node->val);
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            vector<int> res;
            for (auto x : dq) res.push_back(x);
            ans.push_back(res);
            isOrder = !isOrder;
        }
        return ans;
    }
};