Binary Tree Zigzag Level Order Traversal

原题：

题目解析：这个问题的实质是要我们按成访问二叉树的结点，并返回每层访问的结果，这里要求走Z字，其实就是一行正向一行反向。

/*
  the kernel idea is visit a binary search tree in level and 
  the additional work we have to label the end of one level.
*/
vector<vector<int> > zigzagLevelOrder(TreeNode *root) {

	vector<vector<int> > re;
	if(root == NULL)
		return re;

	queue<TreeNode*> visit;
	vector<int> level;

	visit.push(root);
	//NULL stands for the end of this level
	visit.push(NULL);
	//in store every level result, we are asked to change direction.
	bool reback = false;
	while (!visit.empty())
	{
		TreeNode* cur = visit.front();
		
		visit.pop();
		if(cur != NULL){
			if(cur->left)
				visit.push(cur->left);
			if(cur->right)
				visit.push(cur->right);
			level.push_back(cur->val);
		}

		else
		{
			if(reback)
			{
				vector<int> rlevel(level.rbegin(),level.rend());
				re.push_back(rlevel);
				
			}
			else
			re.push_back(level);

			//remember to clear this level.
			level.clear();
			reback = !reback;

			if(!visit.empty())
				visit.push(NULL);
		}
		
	}
	return re;
}