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题目描述
给定一个二叉树,返回其节点值的锯齿形层次遍历。(即先从左往右,再从右往左进行下一层遍历,以此类推,层与层之间交替进行)。
例如:
给定二叉树 [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回锯齿形层次遍历如下:
[
[3],
[20,9],
[15,7]
]
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>>ans;
if(root==NULL)return ans;
queue<TreeNode *>q;
q.push(root);
int flag=1;
while(!q.empty()){
queue<TreeNode *>qt;
vector<int>res;
while(!q.empty()){
TreeNode *t=q.front();q.pop();
res.push_back(t->val);
if(t->left)qt.push(t->left);
if(t->right)qt.push(t->right);
}
if(flag<0)reverse(res.begin(),res.end());
flag=-flag;//标志是否需要反转
ans.push_back(res);
q=qt;
}
return ans;
}
};