java相关-链表-判断环形链表-力扣141

package com.company;

import javax.swing.tree.TreeNode;
import java.lang.reflect.Array;
import java.util.*;

public class Main {

    public static void main(String[] args) {
        ListNode head = new ListNode();
        head.val = 2;


        ListNode second = new ListNode();
        second.val = 3;
        head.next = second;


        ListNode third = new ListNode();
        third.val = 4;
        head.next.next = third;

        ListNode four = new ListNode();
        four.val = 5;
        head.next.next.next = four;

        ListNode five = new ListNode();
        five.val = 6;
        head.next.next.next.next = five;

        ListNode six = new ListNode();
        six.val = 7;
        head.next.next.next.next.next = six;

        six.next=third;//环形指针

        Boolean b=hasCycle(head);
    }

    public static boolean hasCycle(ListNode head) {
        if (head == null) {
            return false;
        }
        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null) {//注意这里判断fast.next如果为null,则fast就是最后一个节点了。最后一个节点就不能再取 fast.next.next
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }


    static class ListNode {
        private Integer val;
        private ListNode next;
    }
}