题解 | #调整牛群顺序#

import java.util.*;

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

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 n int整型 
     * @return ListNode类
     */
    public ListNode moveNthToEnd (ListNode head, int n) {
        // write code here
        ListNode cur = head, p = head;
        for(int i = 1; i <= n-1; i++){
            p = p.next;
        }
        ListNode pre = null;
        while(p.next != null){
            pre = cur;
            p = p.next;
            cur = cur.next;
        }
        if(pre != null){
            p.next = cur;
            pre.next = cur.next;
            cur.next = null;
        }else{
            p.next = cur;
            head = cur.next;
            cur.next = null;
        }
        return head;
    }
}

双指针