[LeetCode] Plus One Linked List

Problem

Given a non-negative integer represented as non-empty a singly linked list of digits, plus one to the integer.

You may assume the integer do not contain any leading zero, except the number 0 itself.

The digits are stored such that the most significant digit is at the head of the list.

Example

Input:
1->2->3

Output:
1->2->4

Solution

class Solution {
    public ListNode plusOne(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode l1 = dummy, l2 = dummy;
        
        //12399456999
        //use l1, l2 to locate 6999
        while (l2.next != null) {
            l2 = l2.next;
            if (l2.val != 9) {
                l1 = l2; //l1 stays at node 6, and l2 gets to the last node
            }
        }
        
        if (l2.val == 9) {
            l1.val++;
            while (l1.next != null) {
                l1.next.val = 0;
                l1 = l1.next;
            }
        } else {
            l2.val++;
        }
        
        if (dummy.val != 0) return dummy;
        return dummy.next;   
    }
}