[LeetCode] Binary Tree Zigzag Level Order Traversal

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]
]

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	vector<vector<int> > ans;
		if ( !root )
			return ans;
		
		int flag=0;

		deque<int> level;
		vector<TreeNode*> cur;
		vector<TreeNode*> next;
		cur.push_back(root);
		while(!cur.empty())
		{
			level.clear();
			next.clear();

			int start=0;
			int end=cur.size();
			while(start!=end)
			{
				TreeNode* tmp=cur[start];
				if ( flag )
					level.push_front(tmp->val);	
				else
					level.push_back(tmp->val);
				if ( tmp->left )
					next.push_back(tmp->left);
				if ( tmp->right)
					next.push_back(tmp->right);
				start++;
			}
			ans.push_back(vector<int>(level.begin(),level.end()));
			cur=next;
			flag=1-flag;
		}
		return ans;
    }
};