Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
For example:
Given binary tree [3,9,20,null,null,15,7]
,
3 / \ 9 20 / \ 15 7
return its level order traversal as:
[ [3], [9,20], [15,7] ]
题意:
二叉树水平遍历
Solution1:Use a queue
code
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public List<List<Integer>> levelOrder(TreeNode root) { 12 List<List<Integer>> result = new ArrayList<>(); 13 //corner 14 if(root ==null) return result; 15 16 Queue<TreeNode> queue = new LinkedList<>(); 17 queue.add(root); 18 while(!queue.isEmpty()){ 19 List<Integer> list = new ArrayList<>(); 20 int size = queue.size(); 21 for(int i = 0; i< size; i++){ 22 TreeNode node = queue.poll(); 23 list.add(node.val); 24 if(node.left!=null){ 25 queue.add(node.left); 26 } 27 if(node.right!=null){ 28 queue.add(node.right); 29 } 30 } 31 result.add(list) ; 32 } 33 return result; 34 } 35 }