1 public class Solution { 2     public ArrayList
      
       
        > levelOrder(TreeNode root) { 3         ArrayList
        
         
          > res = new ArrayList
          
           
            >(); 4 LinkedList
            
              cur = new LinkedList
             
              (); 5 if(root==null) return res; 6 cur.offer(root); 7 while(!cur.isEmpty()){ 8 LinkedList
              
                next = new LinkedList
               
                (); 9 ArrayList
                
                  sol = new ArrayList
                 
                  ();10 for(TreeNode n :cur){11 sol.add(n.val);12 if(n.left!=null) next.offer(n.left);13 if(n.right!=null) next.offer(n.right);14 }15 res.add(sol);16 cur = next;17 }18 return res;19 }20 }