【一天一道LeetCode】#103. Binary Tree Zigzag Level Order Traversal
一天一道LeetCode
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(一)题目
来源: https://leetcode.com/problems/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,null,null,15,7],3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:[
[3],
[20,9],
[15,7]
]
(二)解题
题目大意:给定一个二叉树,按层序遍历输出,层数从1开始,奇数层从左往右输出,偶数层从右往左输出。
解题思路:上一题【一天一道LeetCode】#102. Binary Tree Level Order Traversal采用queue的数据结构来层序输出,每层都是按从左往右的顺序输出,所以,这一题可以采用deque的数据结构,根据奇数和偶数层来判断输出顺序。
详细解释见代码:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
vector<vector<int>> ret;
if(root==NULL) return ret;
deque<TreeNode*> deq;//用来存放每一层的节点
deq.push_back(root);//将根节点放入queue等待处理
int n = 1;//记录层数
while(!deq.empty())
{
vector<int> tempnode;
deque<TreeNode*> temp;//存放下一层的节点
while(!deq.empty()){
if(n%2==1)//奇数层
{
TreeNode* tn = deq.front();//从头开始取节点
tempnode.push_back(tn->val);
deq.pop_front();
if(tn->left!=NULL) temp.push_back(tn->left);//从左往右放入节点
if(tn->right!=NULL) temp.push_back(tn->right);
}
else//偶数层
{
TreeNode* tn = deq.back();//从尾部开始取节点
tempnode.push_back(tn->val);
deq.pop_back();
if(tn->right!=NULL) temp.push_front(tn->right);//从右往左放入节点
if(tn->left!=NULL) temp.push_front(tn->left);
}
}
deq = temp;
ret.push_back(tempnode);
n++;//处理下一层
}
return ret;
}
};
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