Reverse Alternate K Nodes

• 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 and runner->next; i++) runner = runner->next;
    return runner;
}
ListNode* reverse(ListNode* node) {
    ListNode* dummy = new ListNode(-1);
    ListNode* runner = node;
    while(runner) {
        ListNode* nxt = runner->next;
        runner->next = dummy->next;
        dummy->next = runner;
        runner = nxt;
    }
    return dummy->next;
}
ListNode* Solution::solve(ListNode* A, int B) {
    ListNode* dummy = new ListNode(-1);
    dummy->next = A;
    ListNode* tail = dummy;
    while(tail) {
        ListNode* half = cut(tail, B);
        ListNode* safe = half->next;
        half->next = NULL;
        ListNode* rev = reverse(tail->next);
        tail->next = rev;
        while(tail->next) tail = tail->next;
        tail->next = safe;
        for(int i = 0; i < B and tail; i++) tail = tail->next;
    }
    
    return dummy->next;
}