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Question

Given a linked list, reverse the nodes of a linked list k at a time and return its modified 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 nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example,

Given this linked list: 1->2->3->4->5

For $k = 2$, you should return: 2->1->4->3->5

For $k = 3$, you should return: 3->2->1->4->5

Solution

Result: Accepted Time: 8ms

Here should be some explanations.

struct ListNode* reverseKGroup(struct ListNode* head, int k) {
    struct ListNode Head;
    register struct ListNode *now = &Head,*last = &Head,*tmp;
    int cnt = 0;
    Head.next = NULL;
    while(head)
    for(cnt = 0,now = last,last = head; cnt < k &&head; cnt++)
    {
        tmp = head;
        head = head->next;
        tmp -> next = now -> next;
        now->next = tmp;
    }
    if(k != cnt)
    for(head = now->next,now -> next = NULL;head;)
    {
        tmp = head;
        head = head->next;
        tmp -> next = now -> next;
        now->next = tmp;
    }
    return Head.next;
}

Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$