public TreeNode sortedListToBST(ListNode head) {

          if(head==null) return new TreeNode(0);

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

             while(head!=null)

             {

                 arr.add(new TreeNode(head.val));

                 head=head.next;

             }

            return  BST(arr,0,arr.size()-1);

         }

      TreeNode BST(ArrayList<TreeNode> list,int start,int end)

      {

          if(start>=end)

          {

              return list.get(start);

          }

          else

          {

              int mid=start+(end-start)/2;

              TreeNode root=list.get(mid);

              root.left=BST(list,start,mid-1);

              root.right=BST(list,mid+1,end);

              return root;

          }

      }