题解 | #牛群旋转#

{1,2,3,4,5,6,7,8,9},3 假设用例链表长度为9，k=3

从用例中易总结规律得

从头结点，跳 9-3=6次到新头，只要把尾巴连起来，新尾巴断掉即可~

总结一下失败教训

• while xxx!=null 这个循环跳完了 xxx已经是null了不是尾巴啊

import java.util.*;

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

public class Solution {

    public ListNode rotateLeft (ListNode head, int k) {
        if (head == null) return head;
        // 获取长度 count
        int count = 0;
        ListNode cur = head;
        ListNode rightTail = null;
        while (cur != null) {
            count++;
            rightTail = cur; //右边子链表的尾巴节点
            cur = cur.next;
        }
        rightTail.next = head; //首尾连接
        k = k > count ? k % count : k; //减少跳次数
        ListNode newHead = head;
        ListNode leftTail = newHead; //左侧链表的尾巴节点


        for (int i = k; i < count; i++) {
            leftTail = newHead;
            newHead = newHead.next;
        }
        leftTail.next = null; // 断掉尾巴
        return newHead;
    }
}