Day73_LeetCode141.java

/**
 * 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 ;
        }
        ListNode slow = head ;
        ListNode fast = head ;

        while(fast!=null && fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;

            if(fast==slow){
                return true ;
            }
        }
        return false ;
    }
}