<p>Given the <code>root</code> of a binary tree, return <em>the zigzag level order traversal of its nodes&#39; values</em>. (i.e., from left to right, then right to left for the next level and alternate between).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg" style="width: 277px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> [[3],[20,9],[15,7]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1]
<strong>Output:</strong> [[1]]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = []
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

// /**
//  * 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==NULL){
//             return ans;
//         }
//         queue<TreeNode*>q;
//         q.push(root);
//         bool leftToRight=true;
//         while(!q.empty()){
//             vector<int>res;
//              int size=q.size();
//              for(int i=0;i<size;i++){
//                  TreeNode*node=q.front();
//                  q.pop();
//                  if(leftToRight){
//                     res.push_back(node->val);
//                  }
//                  else{
//                      res.insert(res.begin(),node->val);
//                  }
//             if(node->left ){
//                 q.push(node->left);
//             }
//             if(node->right){
//                 q.push(node->right);
//             }
           
//           }
           
//             ans.push_back(res);
//             leftToRight=!leftToRight;
           
//         }
//         return ans;
        
//     }
// };






class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (root == NULL) {
            return ans;
        }
        
        queue<TreeNode*> q;
        q.push(root);
        bool leftToRight = true;
        
        while (!q.empty()) {
            deque<int> res;  
            int size = q.size();
            
            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();
                
                if (leftToRight) {
                    res.push_back(node->val); 
                } else {
                    res.push_front(node->val);  
                }

                if (node->left) {
                    q.push(node->left);
                }
                
                if (node->right) {
                    q.push(node->right);
                }
            }

            ans.push_back(vector<int>(res.begin(), res.end()));  
            leftToRight = !leftToRight;  
        }
        
        return ans;
    }
};

13600