Fast and Slow pointers

hasCycle.cpp

// start with different position
bool hasCycle(ListNode *head) {
    if(!head) return false;
    ListNode *slow = head;
    ListNode *fast = head->next;
    while(fast && fast->next){
        if(slow == fast) return true;
        slow = slow->next;
        fast = fast->next->next;
    }
    return false;
}

// start with same position
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;
}