LeetCode OJ 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]
]

confused what

"{1,#,2,3}"

means? >
read more on how binary tree is serialized on OJ.

class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        ans.clear();
        if (root == NULL) return ans;
        travel(root, 0);
        for (int i = 0; i < ans.size(); i++) {
            if (i % 2) reverse(ans[i].begin(), ans[i].end());
        }
        return ans;
    }
    vector<vector<int> > ans;
    void travel(TreeNode * now, int level) {

        if (level < ans.size()) {
            ans[level].push_back(now->val);
        } else {
            vector<int> v;
            v.push_back(now->val);
            ans.push_back(v);
        }

        if (now->left != NULL) travel(now->left, level + 1);
        if (now->right != NULL) travel(now->right, level + 1);

    }
};