LeetCode OJ Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its level order traversal as:

[  [3],  [9,20],  [15,7]]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

class Solution {public:    vector<vector<int> > levelOrder(TreeNode *root) {        ans.clear();        if (root == NULL) return ans;        travel(root, 0);        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);    }};