Leetcode | Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]

 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 };