Linked List Cycle II - Implementation

Here is the solution:

function detect_cycle(head)
    slow := head
    fast := head
    while fast and fast.next
        slow := slow.next
        fast := fast.next.next
        if slow = fast then
            ptr := head
            while ptr != slow
                ptr := ptr.next
                slow := slow.next
                return ptr
    return null

Time: $O(n)$ where $n$ is the number of nodes. Space: $O(1)$ since you only use two pointers. No visited set needed. The algorithm is simple but powerful.