25. Reverse Nodes in k-Group

Given a linked list, reverse the nodes of a linked listkat a time and return its modified list.

kis 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 ofkthen 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

Fork= 2, you should return:2->1->4->3->5

Fork= 3, you should return:3->2->1->4->5

思路：比较复杂

public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if(head==null||k==1)
            return head;

        ListNode fake = new ListNode(0);
        fake.next = head;
        ListNode pre = fake;
        int i=0;

        ListNode p = head;
        while(p!=null){
            i++;
            if(i%k==0){
                pre = reverse(pre, p.next);
                p = pre.next;
            }else{
                p = p.next; 
            }
        }

        return fake.next; 
    }

/*
 * 0->1->2->3->4->5->6
 * |           |   
 * pre        next
 *
 * after calling pre = reverse(pre, next)
 * 
 * 0->3->2->1->4->5->6
 *          |  |
 *          pre next 
 */
    public ListNode reverse(ListNode pre, ListNode next){
        ListNode last = pre.next;
        ListNode curr = last.next;

        while(curr != next){
            last.next = curr.next;
            curr.next = pre.next;
            pre.next = curr;
            curr = last.next;

        }

        return last; 
    }
}