题解 | #判断链表中是否有环#

用到了双指针

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode removeNthFromEnd (ListNode head, int n) {
        // write code here
        //想法：双指针
        ListNode fast = head;
        ListNode slow = head;
        ListNode test =head;
        ListNode newHead = slow;
        int len = 0;
        //先遍历链表长度，预防n=len而产生的空指针
        while(test != null){
            len++;
            test = test.next;
        }
        
        if(n == len) return head.next;
        
        //fast先遍历n个节点
        for(int i=0;i<n;i++) fast =fast.next;
        
        while(fast.next != null){
            fast = fast.next;
            slow = slow.next;
        }
        
        slow.next = slow.next.next;
        return newHead;
    }
}