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$ curl repovive.com/roadmaps/graph-theory/functional-graphs/linked-list-cycle-ii-implementation

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$ curl repovive.com/roadmaps/graph-theory/functional-graphs/linked-list-cycle-ii-implementation

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Linked List Cycle II - Implementation - Functional Graphs | Graph Theory | Repovive

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Graph Theory37 sections · 1633 units81h total

Open in Course

Linked List Cycle II - Implementation

(Floyd's algorithm in code)

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.