题解 | #旋转链表#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

/**
 * NC211 旋转链表
 * @author d3y1
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @param k int整型
     * @return ListNode类
     */
    public ListNode rotateLinkedList (ListNode head, int k) {
        return solution1(head, k);
        // return solution2(head, k);
    }

    /**
     * 指针
     * @param head
     * @param k
     * @return
     */
    private ListNode solution1(ListNode head, int k){
        if(head == null){
            return null;
        }
        // 统计节点数
        int n = 1;
        ListNode tail = head;
        while(tail.next != null){
            n++;
            tail = tail.next;
        }

        k = k%n;
        if(k == 0){
            return head;
        }
        
        int cnt = n-k;
        ListNode dummyHead = head;
        while(--cnt > 0){
            dummyHead = dummyHead.next;
        }
        
        ListNode pre = dummyHead;
        dummyHead = dummyHead.next;
        pre.next = null;
        tail.next = head;

        return dummyHead;
    }

    /**
     * 双指针: 快慢指针
     * @param head
     * @param k
     * @return
     */
    private ListNode solution2(ListNode head, int k){
        if(head == null){
            return null;
        }
        // 统计节点数
        int n = 1;
        ListNode tail = head;
        while(tail.next != null){
            n++;
            tail = tail.next;
        }

        k = k%n;
        if(k == 0){
            return head;
        }

        ListNode fast = head;
        int step = k;
        while(step-- > 0){
            fast = fast.next;
        }

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

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

        return head;
    }
}