【LeetCode】102. Binary Tree Level Order Traversal-二叉树水平顺序遍历

 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     List<List<Integer>> list1 = new ArrayList<List<Integer>>();
12     Queue<TreeNode> queue =new LinkedList<TreeNode>();
13     
14     public List <List<Integer>> levelOrder(TreeNode root) {
15         if(root==null){
16             return list1;
17         }
18         queue.offer(root);
19         while(!queue.isEmpty()){
20             List<Integer> list2 = new ArrayList<Integer>();
21             int size = queue.size();
22             for(int i=0;i<size;i++){
23                 TreeNode node = queue.remove();
24                 list2.add(node.val);
25                 if(node.left!=null){
26                     queue.offer(node.left);
27                 }
28                 if(node.right!=null){
29                     queue.offer(node.right);
30                 }
31             }
32             list1.add(list2);
33         }
34         return list1;
35     }
36 }