Leetcode 369 Solution

This article provides solution to leetcode question 369 (plus-one-linked-list)
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https://leetcode.com/problems/plus-one-linked-list

Solution

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head)
    {
        ListNode* prev = NULL;
        ListNode* p = head;

        while (p)
        {
            auto next = p->next;
            p->next = prev;
            prev = p;
            p = next;
        }

        return prev;
    }

    ListNode* addone(ListNode* head)
    {
        int carry = 1;

        ListNode* prev = NULL;
        ListNode* p = head;
        while (p)
        {
            p->val += carry;

            if (p->val == 10)
            {
                p->val = 0;
                carry = 1;
            }
            else
            {
                carry = 0;
                break;
            }

            prev = p;
            p = p->next;
        }

        if (carry == 1 && prev)
            prev->next = new ListNode(1);

        return head;
    }

    ListNode* plusOne(ListNode* head)
    {
        if (head == NULL)
            return new ListNode(1);

        head = reverse(head);
        head = addone(head);
        head = reverse(head);
        return head;
    }
};