LeetCode-in-Java

61. Rotate List

Medium

Given the head of a linked list, rotate the list to the right by k places.

Example 1:

Input: head = [1,2,3,4,5], k = 2

Output: [4,5,1,2,3]

Example 2:

Input: head = [0,1,2], k = 4

Output: [2,0,1]

Constraints:

• The number of nodes in the list is in the range [0, 500].
• -100 <= Node.val <= 100
• 0 <= k <= 2 * 109

Solution

import com_github_leetcode.ListNode;

/*
 * 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; }
 * }
 */
public class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null || k == 0) {
            return head;
        }
        ListNode tail = head;
        // find the count and let tail points to last node
        int count = 1;
        while (tail != null && tail.next != null) {
            count++;
            tail = tail.next;
        }
        // calculate number of times to rotate by count modulas
        int times = k % count;
        if (times == 0) {
            return head;
        }
        ListNode temp = head;
        // iterate and go to the K+1 th node from the end or count - K - 1 node from
        // start
        for (int i = 1; i <= count - times - 1 && temp != null; i++) {
            temp = temp.next;
        }
        ListNode newHead = null;
        if (temp != null && tail != null) {
            newHead = temp.next;
            temp.next = null;
            tail.next = head;
        }
        return newHead;
    }
}

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