剑指 Offer 32 - II.从上到下打印二叉树 II

题目描述

原题

从上到下按层打印二叉树,同一层的节点按从左到右的顺序打印,每一层打印到一行。

例如:

给定二叉树: [3,9,20,null,null,15,7],

3
/ \
9 20
/ \
15 7

返回其层次遍历结果:

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

提示:

  1. 节点总数 <= 1000 **

注意:本题与主站 102 题相同:https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

题解

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> levelOrder = new ArrayList<>();
if(root == null){
return levelOrder;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while(!queue.isEmpty()){
List<Integer> level = new ArrayList<>();
int size = queue.size();
for(int i = 0;i<size;i++){
TreeNode node = queue.poll();
level.add(node.val);
if(node.left!=null){
queue.offer(node.left);
}
if(node.right!=null){
queue.offer(node.right);
}
}
levelOrder.add(level);
}
return levelOrder;
}
}