102. 二叉树的层次遍历

 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 class Solution {
11     List<List<Integer>> levels = new ArrayList<List<Integer>>();
12 
13     public void DFS(TreeNode node, int level) {
14         if (levels.size() <= level) {
15             levels.add(new ArrayList<>());
16         }
17 
18         levels.get(level).add(node.val);
19         if (node.left != null)
20             DFS(node.left, level + 1);
21         if (node.right != null)
22             DFS(node.right, level + 1);
23     }
24     
25     public List<List<Integer>> levelOrder(TreeNode root) {
26         if (root == null) return levels;
27         DFS(root, 0);
28         return levels;
29     }
30 }