369. 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]

time: O(n), space: O(1)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode plusOne(ListNode head) {
        if(head == null) {
            return head;
        }
        ListNode newHead = reverse(head);
        ListNode cur = newHead;
        while(cur.next != null) {
            if(cur.val < 9) {
                cur.val += 1;
                return reverse(newHead);
            }
            cur.val = 0;
            cur = cur.next;
        }
        if(cur.val < 9) {
            cur.val += 1;
            return reverse(newHead);
        }
        cur.val = 0;
        cur.next = new ListNode(1);
        return reverse(newHead);
    }
    
    public ListNode reverse(ListNode head) {
        ListNode prev = null, cur = head;
        while(cur != null) {
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        return prev;
    }
}