/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode plusOne(ListNode head) {
        if(head == null) return head;
        ListNode l = new ListNode(0);
        ListNode dummy = l;
        ListNode reHead = reverseList(head);
        int carry = 1;
        int digit = 0;
        ListNode cur = reHead;
        while(cur != null || carry != 0){
            int index = (cur == null)?  0 : cur.val ;
            digit = (index + carry) % 10;
            carry = (index + carry) / 10;
            l.next = new ListNode(digit);
            l = l.next;
            if(cur != null)
                cur = cur.next;
        }
        return reverseList(dummy.next);
    }
    public ListNode reverseList(ListNode head){
        if(head == null || head.next == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = head.next;
        while(cur != null){
            head.next = cur.next;
            cur.next = dummy.next;
            dummy.next = cur;
            cur = head.next;
        }
        return dummy.next;
    }
}