K reverse linked list

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* cut(ListNode* A, int k) {
    ListNode* runner = A;
    for(int i = 0; i < k - 1; i++) runner = runner->next;
    return runner;
}
ListNode* reverse(ListNode* A) {
    ListNode* dummy = new ListNode(-1);
    ListNode* runner = A;
    while(runner) {
        ListNode* next = runner->next;
        runner->next = dummy->next;
        dummy->next = runner;
        runner = next;
    }
    return dummy->next;
}
ListNode* Solution::reverseList(ListNode* A, int B) {
    ListNode* dummy = new ListNode(-1);
    ListNode* tail = dummy;
    ListNode* runner = A;
    while(runner) {
        ListNode* last = cut(runner, B);
        ListNode* next = last->next;
        last->next = NULL;
        ListNode* rev = reverse(runner);
        tail->next = rev;
        tail = runner;
        runner = next;
    }
    return dummy->next;
}