LeetCode25 Reverse Nodes in k-Group

题目：

       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.

 

public class ReverseNodesKGroup {
    public ListNode reverseKGroup(ListNode head, int k) {

        if(head==null||head.next==null||k==1)
            return head;

        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode last = dummy;
        ListNode pre = dummy.next;

        int len = getLength(head);

        while(len>=k){
            for(int i=0;i<(k-1);i++){
                ListNode cur = pre.next;
                pre.next = cur.next;
                cur.next = last.next;
                last.next = cur;
            }
            len =len - k;
            last = pre;
            pre = pre.next;
        }
        return dummy.next;
    }

    public int getLength(ListNode head){
        int len = 0;
        while(head!=null){
            len++;
            head = head.next;
        }
        return len;
    }
}