A classic interview question of linked list. The idea is to maintain two pointers, one move one step at a time and the other move two steps at a time. If they become the same, a cycle exists. Otherwise, they will become NULL finally and there is no cycle.
The code is as follows.
1 bool hasCycle(ListNode *head) { 2 ListNode* slow = head; 3 ListNode* fast = head; 4 while (fast && fast -> next) { 5 slow = slow -> next; 6 fast = fast -> next -> next; 7 if (slow == fast) return true; 8 } 9 return false; 10 }
