# LeetCode 103: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]
]


confused what "{1,#,2,3}" means?

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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);
}
};


posted @ 2017-08-20 20:51 mfmdaoyou 阅读(...) 评论(...) 编辑 收藏