141. 链表是否成环 Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

题意：判断链表是否成环。

注意：最后一个节点的next指针，有可能指向链表中任意一个节点

方法一：使用HashSet，保存遍历过的节点的HashCode

public class Solution {
    public bool HasCycle(ListNode head) {
		if (head == null) {
			return false;
		}
		ListNode node = head;
		HashSet<int> hashSet = new HashSet<int>();
		hashSet.Add(node.GetHashCode());
		while (node.next != null) {
			node = node.next;
			int hash = node.GetHashCode();
			if (hashSet.Contains(hash)) {
				return true;
			} else {
				hashSet.Add(hash);
			}
		}
		return false;
    }
}

方法二：使用两个指针，一个快一个慢，如果两个指针相遇，则链表成环

public class Solution {
    public bool HasCycle(ListNode head) {
		if (head == null || head.next == null) {
			return false;
		}
		ListNode slow = head;
		ListNode fast = head;
		while (fast.next != null && fast.next.next != null) {
			slow = slow.next;
			fast = fast.next.next;
			if (slow == fast) {
				return true;
			}
		}
		return false;
    }
}

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