25. Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is.

You may not alter the values in the list’s nodes, only nodes themselves may be changed.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        if(!head) return nullptr;
        int n = k;
        ListNode* dummy = new ListNode();
        ListNode* tmp;
        ListNode* tail = head;
        while(n--) {
            tmp = head->next;
            head->next = dummy->next;
            dummy->next = head;
            head = tmp;
            if(!head && n) {
                return reverseKGroup(dummy->next, k - n);
            }
        }
        if(tmp) {
            tail->next = reverseKGroup(tmp, k);
        }
        return dummy->next;
    }
};