C++

1class Solution {
2  public:
3    bool hasCycle(ListNode *head) {
4      if (!head) return false;
5      if (!head->next) return false;
6
7      ListNode *tortoise = head->next;
8      ListNode *hare = head->next->next;
9
10      for(int i=0; i<10001; ++i) {
11        if (tortoise == hare) return true;
12        tortoise = tortoise->next;
13        if (!tortoise) return false;
14        hare = hare->next;
15        if(!hare || !hare->next) return false;
16        hare = hare->next;
17      }
18
19      return false;
20    }
21};

Javascript

1var hasCycle = function(head) {
2  if (!head || !head.next) return false;
3
4  let tortoise = head.next;
5  let hare = head.next.next;
6
7  for(let i=0; i<10001; ++i) {
8    if (tortoise === hare) return true;
9    tortoise = tortoise.next;
10    if(!tortoise) return false;
11    hare = hare.next;
12    if(!hare || !hare.next) return false;
13    hare = hare.next;
14  }
15
16  return false;
17};

Ruby

1def hasCycle(head)
2  return false if !head or !head.next
3
4  tortoise = head.next
5  hare = head.next.next
6
7  0.upto(10000) do |_|
8    return true if tortoise == hare
9
10    tortoise = tortoise.next
11    return false if !tortoise
12
13    hare = hare.next
14    return false if !hare or !hare.next
15    hare = hare.next
16  end
17
18  false
19end