LeetCode141. Linked List Cycle

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本文介绍了一种使用HashSet来判断链表是否存在循环的方法，并提供了一种更高效的空间复杂度为O(1)的解决方案——快慢指针追及问题。

题目

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

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

解答

一般思路：HashSet

public class Solution {
    public boolean hasCycle(ListNode head) {
        HashSet<ListNode> set = new HashSet<ListNode>();
        while (head != null) {
            if (set.contains(head)) return true;
            set.add(head);
            head = head.next;
        }
        return false;
    }
}

without using extra space：追及问题

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) return false;
        ListNode slow = head, fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null) return false;
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }
}