141. Linked List Cycle

https://leetcode.com/problems/linked-list-cycle/

Approach 1: Hash Table

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null||head.next==null) return false;
        Set<ListNode> nodeSeen=new HashSet<>();
        while(!nodeSeen.contains(head)){
            nodeSeen.add(head);
            head=head.next;
            if(head==null) return false;
        }
        return true;
    }
}

Approach 2: Two Pointers

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