Reverse a singly linked list

Junior PRO

Task

final class ListNode {
    var value: Int
    var next: ListNode?
    init(_ value: Int) { self.value = value }
}

Reverse the list 1 → 2 → 3 → 4 into 4 → 3 → 2 → 1. Iteratively, in O(n) time and O(1) memory.

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