[Linked list] Determine whether the linked list has a cycle

Double pointer:

Use two pointers fast and slow, both of which start at the head of the linked list.

The slow moves backward one position at a time, while the fast pointer moves two positions at a time.

If there is a ring in the linked list, the fast pointer will eventually meet the slow pointer in the ring.

prove:

 

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