103

/**
 * 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) {
        if(!root) return {};
        queue<TreeNode*> q;
        vector< vector<int> > res;
        q.push(root);
        int level = 0;

        while(!q.empty()){
            int sz = q.size();
            vector<int> curr(sz);
            for(int i=0; i<sz; i++){
                TreeNode *front = q.front();
                q.pop();
                if(level) curr[sz-i-1] = front->val;
                else curr[i] = front->val;
                if(front->left) q.push(front->left);
                if(front->right) q.push(front->right);
            }
            res.push_back(curr);
            level = !level;
        }
        return res;
    }
};