Plus One Linked List

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

/**
 * 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 new ListNode(1);
        if (head.next == null) {
            if (head.val < 9) {
                head.val += 1;
                return head;
            } else {
                head.val = 0;
                ListNode node1 = new ListNode(1);
                node1.next = head;
                return node1;
            }
        }

        ListNode reverseNode = reverseLinkedList(head);
        int carry = 1;
        ListNode runner = reverseNode;

        while (runner.next != null) {
            int current = runner.val;
            int total = current + carry;
            runner.val = total % 10;
            carry = total / 10;
            runner = runner.next;
        }

        if (carry == 1 && runner.val == 9) {
            runner.val = 0;
            ListNode node1 = new ListNode(1);
            runner.next = node1;
        } else {
            runner.val += carry;
        }


        return reverseLinkedList(reverseNode);
    }

    private ListNode reverseLinkedList(ListNode node) {
        if (node == null) return null;
        if (node.next == null) return node;

        ListNode next = node.next;
        ListNode reverse = reverseLinkedList(next);
        next.next = node;
        node.next = null;
        return reverse;
    }

}