LeetCode 103 Binary Tree Zigzag Level Order Traversal

题目

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]
]

广度遍历，每层翻转

思路

1 Collections.reverse()这个函数真好！

2 广度遍历是基础的基础，要记得多练习。

代码

public class Solution {
public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
if(root==null){
return ans;
}
ArrayList<Integer> temp = new ArrayList<Integer>();
LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
boolean reverse = false;
int num = 0;
int count =1;
while(!queue.isEmpty()){
TreeNode cur = queue.remove();
temp.add(cur.val);
count--;
if(cur.left!=null){
queue.add(cur.left);
num++;
}
if(cur.right!=null){
queue.add(cur.right);
num++;
}
if(count==0){
count=num;
num=0;
if(reverse){
Collections.reverse(temp);
reverse=false;
}
else{
reverse=true;
}
ans.add(new ArrayList<Integer>(temp));
temp.clear();
}
}
return ans;
}
}