leetcode-Java-141. Linked List Cycle.java

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

HashSet<ListNode> set = new HashSet<ListNode>();

while(head!=null) {
if(set.contains(head)) {
return true;
}
set.add(head);
head = head.next;
}
return false;
}
}

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

ListNode fast = head,
slow = head;

while(true) {
if(fast.next != null && fast.next.next!=null) {
fast = fast.next.next;
slow = slow.next;
}else{
return false;
}
if(slow == fast) {
return true;
}

}
}
}