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]
]

class Solution {
public:
    void reverse(vector<int> &arr) {
        for (int i = 0, j = arr.size() - 1; i < j; i++, j--) swap(arr[i], arr[j]);
    }
    
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int> > ans;
        if (root == NULL) return ans;
        vector<vector<TreeNode*> > layers(2);
        vector<int> tmp;
        int cur = 0, next = 1;
        layers[cur].push_back(root);
        
        while (!layers[cur].empty()) {
            layers[next].clear();
            tmp.clear();
            for (auto node : layers[cur]) {
                if (node->left) layers[next].push_back(node->left);
                if (node->right) layers[next].push_back(node->right);
                tmp.push_back(node->val);
            }
            if (cur == 1) reverse(tmp);
            ans.push_back(tmp);
            cur = !cur, next = !next;
        }
        
        return ans;
    }
};