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.
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
Note:
Only constant extra memory is allowed.
You may not alter the values in the list's nodes, only nodes itself may be changed.
Solution in C++
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: ListNode* reverseKGroup(ListNode* head, int k) { vector<ListNode *> p(k); ListNode *h=head; int i=0; while(i<k) { if (!h) return head; p[i++]=h; h=h->next; } i--; while(i>0) { p[i]->next=p[i-1]; i--; } p[0]->next=(h ? reverseKGroup(h, k): NULL); return p[k-1]; } };