Binary Search Tree BST Template

Use one queue + size variable

 1 public class Solution {
 2     public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
 3         ArrayList result = new ArrayList();
 4         if (root == null)
 5             return result;
 6         Queue<TreeNode> queue = new LinkedList<TreeNode>();
 7         queue.offer(root);
 8
 9         while (!queue.isEmpty()) {
10             int size = queue.size();
11             ArrayList<Integer> level = new ArrayList<Integer>();
12             for (int i = 0; i < size; i++) {
13                 TreeNode head = queue.poll();
14                 level.add(head.val);
15                 if (head.left != null)
16                     queue.offer(head.left);
17                 if (head.right != null)
18                     queue.offer(head.right);
19             }
20             result.add(level);
21         }
22         return result;
23     }
24 }