Leetcode | Binary Tree Zigzag Level Order Traversal

 1 class Solution {
 2 public:
 3     void reverse(vector<int> &arr) {
 4         for (int i = 0, j = arr.size() - 1; i < j; i++, j--) swap(arr[i], arr[j]);
 5     }
 6     
 7     vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
 8         vector<vector<int> > ans;
 9         if (root == NULL) return ans;
10         vector<vector<TreeNode*> > layers(2);
11         vector<int> tmp;
12         int cur = 0, next = 1;
13         layers[cur].push_back(root);
14         
15         while (!layers[cur].empty()) {
16             layers[next].clear();
17             tmp.clear();
18             for (auto node : layers[cur]) {
19                 if (node->left) layers[next].push_back(node->left);
20                 if (node->right) layers[next].push_back(node->right);
21                 tmp.push_back(node->val);
22             }
23             if (cur == 1) reverse(tmp);
24             ans.push_back(tmp);
25             cur = !cur, next = !next;
26         }
27         
28         return ans;
29     }
30 };