1.NC4 判断链表中是否有环

1.NC4 判断链表中是否有环

判断链表中是否有环_牛客题霸_牛客网 (nowcoder.com)

解法1

经典的解法是使用双指针-快慢指针，快指针走两步，满指针走一步，如果两个指针刚好相遇，那么链表存在环。

import java.util.*;
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                return true;
            }
        }
        return false;
    }
}

解法2

使用HashSet，set是去重的，每次add(node)之前和之后记录set的长度，如果两次的长度一样说明没有添加成功，那么链表中有环

import java.util.*;
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        HashSet set = new HashSet<>();
        ListNode node = head;
        while(node != null){
            int len1 = set.size();
            set.add(node);
            int len2 = set.size();
            if(len1 == len2){
                return true;
            }else   node = node.next;
        }
        return false;
    }
}