25. Reverse Nodes in k-Group

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

 

Example 1:

Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

Example 2:

Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]

Example 3:

Input: head = [1,2,3,4,5], k = 1
Output: [1,2,3,4,5]

Example 4:

Input: head = [1], k = 1
Output: [1]

 

Constraints:

  • The number of nodes in the list is in the range sz.
  • 1 <= sz <= 5000
  • 0 <= Node.val <= 1000
  • 1 <= k <= sz

Solution

/**
 * 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:
    bool count(ListNode *head, int k)
    {
        ListNode *temp = head;
        int cnt = 0;
        while (temp != NULL)
        {
            cnt++;
            temp = temp->next;
        }
        cout << cnt << " ";
        if (cnt >= k)
        {
            return true;
        }
        return false;
    }
    ListNode *reverseKGroup(ListNode *head, int k)
    {
        if (head == NULL || head->next == NULL)
        {
            return head;
        }
        ListNode *pre = NULL;
        ListNode *curr = head;
        int t = k;
        ListNode *temp;
        while (curr != NULL && t--)
        {
            temp = curr->next;
            curr->next = pre;
            pre = curr;
            curr = temp;
        }
        if (temp != NULL)
        {
            if (count(temp, k))
            {
                head->next = reverseKGroup(curr, k);
            }
            else
            {
                head->next = curr;
            }
        }

        return pre;
    }
};