Binary Search Tree BST Template

Use one queue + size variable

 public class Solution {
     public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
         ArrayList result = new ArrayList();
         if (root == null)
             return result;
         Queue<TreeNode> queue = new LinkedList<TreeNode>();
         queue.offer(root);
 
         while (!queue.isEmpty()) {
             int size = queue.size();
             ArrayList<Integer> level = new ArrayList<Integer>();
             for (int i = 0; i < size; i++) {
                 TreeNode head = queue.poll();
                 level.add(head.val);
                 if (head.left != null)
                     queue.offer(head.left);
                 if (head.right != null)
                     queue.offer(head.right);
             }
             result.add(level);
         }
         return result;
     }
 }