102. Binary Tree Level Order Traversal Java Solutions

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},

return its level order traversal as:

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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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