141

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public bool HasCycle(ListNode head) {
        var slow = head; 
        var fast = head;
 
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
 
            if (slow == fast) return true;
        }
 
        return false;
    }
}

Once both pointers are inside the cycle, fast gains exactly one node on slow per step, so the gap between them shrinks by 1 each step (…, 2, 1, 0). It can’t jump over 0, so they must land on the same node. If there’s no cycle, fast just hits null first.