给定一个链表，判断链表中是否有环。
为了表示给定链表中的环，我们使用整数 pos 来表示链表尾连接到链表中的位置（索引从 0 开始）。 如果 pos 是 -1，则在该链表中没有环。
示例 1：
输入：head = [3,2,0,-4], pos = 1
输出：true
解释：链表中有一个环，其尾部连接到第二个节点。
哈希表
=============================================Python=========================================
# Definition for singly-linked list. # class ListNode: # def __init__(self, x): # self.val = x # self.next = None class Solution: def hasCycle(self, head: ListNode) -> bool: s = set() if not head: return False while head: if head not in s: s.add(head) head = head.next else: return True return False
=========================================Java=======================================
/** * 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) { HashSet<ListNode> s = new HashSet<>(); if (head == null) { return false; } while (head != null) { if (s.contains(head) == false) { s.add(head); head = head.next; } else { return true; } } return false; } }
=========================================Go==================================
/** * Definition for singly-linked list. * type ListNode struct { * Val int * Next *ListNode * } */ func hasCycle(head *ListNode) bool { m := make(map[*ListNode]int) for head != nil { if _, ok := m[head]; ok{ return true } m[head] = 1 head = head.Next } return false }
双指针
=============================================Python=========================================
# Definition for singly-linked list. # class ListNode: # def __init__(self, x): # self.val = x # self.next = None class Solution: def hasCycle(self, head: ListNode) -> bool: fast = slow = head if not head: return False while fast and fast.next: fast = fast.next.next slow = slow.next if slow == fast: return True return False
原文：https://www.cnblogs.com/liushoudong/p/13496845.html