369. 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

Analysis:
Two pointers. i reaches the last node before 9, j reaches the tail node.
1-> 2 -> 4 -> 9 -> null
              i                 j
We can set i.val++, and set node after i's val to 0.
One corner case is the list starts with 9 and only has 9. like  9 -> 9->null. So we start checking 9 from dummy.

Solution:

class Solution {
    public ListNode plusOne(ListNode head) {
        if (head == null)
            return null;

        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode i = dummy;
        ListNode j = head;
        while (j != null) {
            if (j.val != 9) {
                i = j;
            }
            j = j.next;
        }
        i.val += 1;
        i = i.next;
        while (i != null) {
            i.val = 0;  
            i = i.next;
        }
        return dummy.val == 0 ? dummy.next : dummy;
    }
}