RotateList.java

// https://leetcode.com/problems/rotate-list/
// #linked-list #two-pointer
/**
 * Definition for singly-linked list. public class ListNode { int val; ListNode next; ListNode() {}
 * ListNode(int val) { this.val = val; } ListNode(int val, ListNode next) { this.val = val;
 * this.next = next; } }
 */
class Solution {
  public ListNode rotateRight(ListNode head, int k) {
    if (head == null) {
      return head;
    }

    int len = 0;
    ListNode tmp = head;
    while (tmp != null) {
      len += 1;
      tmp = tmp.next;
    }

    int left_len = len - (k % len);
    ListNode left_tail = head;
    while (left_len > 1) {
      left_tail = left_tail.next;
      left_len -= 1;
    }
    ListNode new_head = left_tail.next;
    left_tail.next = null;
    ListNode right_tail = new_head;
    if (right_tail != null) {
      while (right_tail.next != null) {
        right_tail = right_tail.next;
      }
      right_tail.next = head;
    } else {
      new_head = head;
    }

    return new_head;
  }
}