[LeetCode] Binary Tree Zigzag Level Order Traversal

vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
	vector<TreeNode*> last_level_node;
	vector<vector<int>> val_in_level;
	if (root == NULL)
	{
		return val_in_level;
	}
	last_level_node.push_back(root);

	vector<TreeNode*> cur_level_node;
	bool zigzag = true;
	do
	{
		cur_level_node.clear();
		vector<int> val;		
		for (int i = 0; i < last_level_node.size(); i++)
		{
			TreeNode* cur_node = last_level_node[i];
			TreeNode* left_node = NULL;
			TreeNode* right_node = NULL;
			if(cur_node != NULL)
			{
				left_node = cur_node->left;
				right_node = cur_node->right;
				if (zigzag)
				{
					val.push_back(cur_node->val);
				} 
				else
				{
					val.insert(val.begin(), cur_node->val);
				}
				if (left_node != NULL)
				{
					cur_level_node.push_back(left_node);
				}
				if (right_node != NULL)
				{
					cur_level_node.push_back(right_node);
				}		
			}
		}
		val_in_level.push_back(val);
		zigzag = !zigzag;
		last_level_node = cur_level_node;
	}while (!cur_level_node.empty());
	return val_in_level;

}

思路和Binary Tree Level Order Traversal类似，只是多加一个标识，保证前一次正向存数值，下一次则反向存数值；前一次反向存数值，下一次则正向存数值