61. Rotate List

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

time: O(n), space: O(1)

/**
 * 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) {
        if(head == null) {
            return head;
        }
        int len = 0;
        ListNode cur = head;
        while(cur != null) {
            cur = cur.next;
            len++;
        }
        k = k % len;
        
        ListNode p1 = head, p2 = head;
        for(int i = 0; i < k; i++) {
            p1 = p1.next;
        }
        while(p1.next != null) {
            p1 = p1.next;
            p2 = p2.next;
        }
        p1.next = head;
        head = p2.next;
        p2.next = null;
        
        return head;
    }
}