public ListNode toLinkedList(ArrayList<Integer> nums) {  
        ListNode head  = null;
        ListNode dummy = null;
        if(nums.size()==0) return null;

        for(int i=0;i<nums.size();i++){
            if(head==null){
            head = new ListNode(nums.get(i));
            dummy = head;
            }
            else{
                dummy.next = new ListNode(nums.get(i));
                dummy = dummy.next;
            }
        }
        return head;
    }