[LeetCode] Linked List Cycle

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.

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