141. 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?

思路1：naive思路用Hashset，看看重复了没有。但是需要extra space

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

        return false;
    }
}

思路2：用双指针，一快一慢。想像他们是在一个操场跑步，如果有圈总会遇上

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head==null){
            return false;
        }
        ListNode walk = head;
        ListNode run = head;

        while(run.next!=null&&run.next.next!=null){
            walk = walk.next;
            run = run.next.next;
            if (walk==run){
                return true;
            }
        }

        return false;
    }
}