LeetCode 103:Binary Tree Zigzag Level Order Traversal

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本文介绍了一种解决二叉树锯齿形层序遍历问题的方法，通过递归方式实现节点值的从左至右再从右至左的交替层次遍历，并给出具体实例。

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},

return its zigzag level order traversal as:

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

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

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 //递归方法：用一个bool记录是从左到右还是从右到左，每一层结束就翻转一下。
 class Solution {
 public:
	 vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
		 vector<vector<int>> res;
		 traverse(root, 1, res, true);
		 return res;
	 }

	 void traverse(TreeNode* root, int level, vector<vector<int>> &res, bool left_to_right){
		 if (!root) return;
		 if (level > res.size())
			 res.push_back(vector<int>());
		 if (left_to_right)
			 res[level - 1].push_back(root->val);
		 else
			 res[level - 1].insert(res[level - 1].begin(), root->val);

		 traverse(root->left, level + 1, res, !left_to_right);
		 traverse(root->right, level + 1, res, !left_to_right);
	 }
 };