[LeetCode] Problem 61 - Rotate List

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

Example

No.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

No.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

Code

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public class ListNode {
    int val;
    ListNode next = null;
    ListNode(int x) { val = x; }
}
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public ListNode rotateRight(ListNode head, int k) {
    if (head == null)
        return null;
    
    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode slow = dummy;
    ListNode fast = dummy;
    
    int length = getLength(head);
    k %= length;

    for (int i = 0; i < k; i++)
        fast = fast.next;

    while (fast.next != null) {
        fast = fast.next;
        slow = slow.next;
    }

    fast.next = head;
    dummy.next = slow.next;
    slow.next = null;

    return dummy.next;
}

private int getLength(ListNode head) {
    int count = 0;

    while (head != null) {
        head = head.next;
        count++;
    }

    return count;
}