linked-list-cycle.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode(int x) : val(x), next(NULL) {}
 * };
 */

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