Linked list cycle

Hash Table Linked List Two Pointers

53.9% acceptance

Jan 13, 2026

17412

1574

Given head, the head of a linked list, determine if the linked list has a cycle in it. Return true if there is a cycle in the linked list. Otherwise, return false.

Solution

C++

Time O(n)

Space O(1)

LeetCode

solution.cpp

class Solution {
public:
  bool hasCycle(ListNode *head) {
    ListNode *slow = head, *fast = head;
    while (fast && fast->next) {
      slow = slow->next;
      fast = fast->next->next;
      if (slow == fast) return true;
    }
    return false;
  }
};