Linked List Cycle Leetcode

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

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

 

这道题很简单也很经典感觉。一开始用hashset做的，但这样就用了extra space了。后来发现可以用双指针。

hashset

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

双指针

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

双指针是公认的解法，这次都忘记了，下次要记住。。。