3
   / \
  9  20
    /  \
   15   7

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> levelOrder = new ArrayList<List<Integer>>();
        if(root == null){
            return levelOrder;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> level = new ArrayList<Integer>();
            int size = queue.size();
            for(int i = 0; i < size;i++){
                TreeNode node = queue.poll();
                level.add(node.val);
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
            levelOrder.add(level);
        }
        return levelOrder;
    }
}