[LeetCode] 369. Plus One Linked List

 Problem : https://leetcode.com/problems/plus-one-linked-list/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode plusOne(ListNode head) {
        // reverse list
        ListNode reversed = reverse(head);
        
        int carry = 1;    
        ListNode p = reversed;
        ListNode last = null;
        
        while(p != null) {
            int total = p.val + carry;
            p.val = total % 10;
            carry = total / 10;
            last = p;
            p = p.next;
        }
        
        if (carry != 0) {
            last.next = new ListNode(carry);
        }
        
        
        return reverse(reversed);
    }
    
    ListNode reverse(ListNode head) {
        ListNode last = null;
        ListNode current = head;
        ListNode next = head.next;
        
        while (next != null) {
            current.next = last;
            last = current;
            current = next;
            next = current.next;
        }
        
        // remember to link the last element!
        current.next = last;
        return current;
    }
}