Answer by Neil for How to detect a loop in a linked list?

In this context, there are loads to textual materials everywhere. I just wanted to post a diagrammatic representation that really helped me to grasp the concept.

When fast and slow meet at point p,

Distance travelled by fast = a+b+c+b = a+2b+c

Distance travelled by slow = a+b

Since the fast is 2 times faster than the slow. So a+2b+c = 2(a+b), then we get a=c.

So when another slow pointer runs again from head to q, at the same time, fast pointer will run from p to q, so they meet at the point q together.

public ListNode detectCycle(ListNode head) {
    if(head == null || head.next==null)
        return null;

    ListNode slow = head;
    ListNode fast = head;

    while (fast!=null && fast.next!=null){
        fast = fast.next.next;
        slow = slow.next;

        /*
        if the 2 pointers meet, then the 
        dist from the meeting pt to start of loop 
        equals
        dist from head to start of loop
        */
        if (fast == slow){ //loop found
            slow = head;
            while(slow != fast){
                slow = slow.next;
                fast = fast.next;
            }
            return slow;
        }            
    }
    return null;
}