61. Rotate List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
      
      ListNode dummy = new ListNode(-1);
      dummy.next = head;
      
      // get the length of the linked list 
      int length = 1;
      
      while( head != null && head.next != null ){
        head = head.next;
        length++;
      }
      
      k = k % length;
      
      // move the fast and slow pointers to the  desired position 
      ListNode slow = dummy;
      ListNode fast = dummy;
      
      while( k > 0){
        fast = fast.next;
        k--;
      }
      
      while( fast != null  && fast.next != null ){
        fast = fast.next;
        slow = slow.next;
      }
      
      fast.next = dummy.next;
      dummy.next = slow.next;
      slow.next = null;
      return dummy.next;
    }
}

 

Given a linked list, rotate the list to the right by k places, where k is non-negative.

Example 1:

Input: 1->2->3->4->5->NULL, k = 2
Output: 4->5->1->2->3->NULL
Explanation:
rotate 1 steps to the right: 5->1->2->3->4->NULL
rotate 2 steps to the right: 4->5->1->2->3->NULL

Example 2:

Input: 0->1->2->NULL, k = 4
Output: 2->0->1->NULL
Explanation:
rotate 1 steps to the right: 2->0->1->NULL
rotate 2 steps to the right: 1->2->0->NULL
rotate 3 steps to the right: 0->1->2->NULL
rotate 4 steps to the right: 2->0->1->NULL