ctci/02. Linked lists/hackerrank/has_cycle.java

/*
Detect a cycle in a linked list. Note that the head pointer may be 'null' if the list is empty.

A Node is defined as: 
    class Node {
        int data;
        Node next;
    }
*/

class Node {
    int data;
    Node next;
}

boolean hasCycle(Node head) {
    Node slow = head;
    Node fast = head;
    while (fast != null && fast.next != null){
        slow = slow.next;
        fast = fast.next.next;
        if(slow == fast) return true;
    }
    return false;
}