Leetcode103.Binary_Tree_Zigzag_Level_Order_Traversal

用一个栈来使数据反向。
时间复杂度：O(N)
C++代码：

class Solution {
public:
	vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
		if (root == nullptr)
			return {};
		int dir = 1;
		vector<vector<int>> result(1);
		queue<TreeNode*> record;
		stack<TreeNode*> reverse;
		record.push(root);
		while (!record.empty())
		{
			TreeNode* temp = record.front();
			record.pop();
			(*(result.end() - 1)).push_back(temp->val);
			if (dir)
			{
				if (temp->left != nullptr)
					reverse.push(temp->left);
				if (temp->right != nullptr)
					reverse.push(temp->right);
			}
			else
			{
				if (temp->right != nullptr)
					reverse.push(temp->right);
				if (temp->left != nullptr)
					reverse.push(temp->left);
			}
			if (record.empty())
			{
				dir = !dir;
				if (!reverse.empty())
					result.push_back({});
				while (!reverse.empty())
				{
					TreeNode* now = reverse.top();
					reverse.pop();
					record.push(now);
				}
			}
		}
		return result;
	}
};