public class Solution {

public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {

// Start typing your Java solution below

// DO NOT write main() function

ArrayList<ArrayList<Integer>> arr = new ArrayList<ArrayList<Integer>>();

if(root==null) return arr;

Queue<TreeNode> qc = new LinkedList<TreeNode>();

Queue<TreeNode> qn = new LinkedList<TreeNode>();

ArrayList<Integer> al = new ArrayList<Integer>();

qn.offer(root);

TreeNode c;

while(true){

if(qc.isEmpty() && qn.isEmpty()){

if(al.size()!=0){

arr.add(new ArrayList<Integer>(al));

al.clear();

}

break;

}

if(qc.isEmpty()){

Queue<TreeNode> tmp = qc;

qc = qn;

qn = tmp;

if(al.size()!=0){

arr.add(new ArrayList<Integer>(al));

al.clear();

}

}else{

c = qc.poll();

if(c.left!=null) qn.offer(c.left);

if(c.right!=null) qn.offer(c.right);

al.add(c.val);

}

}

return arr;

}

}