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,#,#,15,7},
3
/
9 20
/
15 7
return its level order traversal as:
[ [3], [9,20], [15,7] ]
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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 public class Solution { 11 public List<List<Integer>> levelOrder(TreeNode root) { 12 List<List<Integer>> res = new ArrayList<List<Integer>>(); 13 Queue<TreeNode> q = new LinkedList<TreeNode>(); 14 if(root == null) return res; 15 q.add(root); 16 while(!q.isEmpty()){ 17 List<Integer> tmp = new ArrayList<Integer>(); 18 int size = q.size(); 19 for(int i = 0; i< size; i++){ 20 TreeNode node = q.poll(); 21 tmp.add(node.val); 22 if(node.left != null) q.add(node.left); 23 if(node.right != null) q.add(node.right); 24 } 25 res.add(tmp); 26 27 } 28 return res; 29 } 30 }
java 中ｑｕｅｕｅ 的操作：
add 增加一个元索 如果队列已满，则抛出一个IIIegaISlabEepeplian异常
remove 移除并返回队列头部的元素 如果队列为空，则抛出一个NoSuchElementException异常
element 返回队列头部的元素 如果队列为空，则抛出一个NoSuchElementException异常
offer 添加一个元素并返回true 如果队列已满，则返回false
poll 移除并返问队列头部的元素 如果队列为空，则返回null
peek 返回队列头部的元素 如果队列为空，则返回null
put 添加一个元素 如果队列满，则阻塞
take 移除并返回队列头部的元素 如果队列为空，则阻