Return the head node of the singly linked list with each pair of nodes swapped. If there is a last odd node leave it in place. Example: Input: 1 -> 2 -> 3 -> 4 -> 5 Output: 2 -> 1 -> 4 -> 3 -> 5

linkedListSwapElements.swift

enum LinkedList {

    case empty
    indirect case node(T, LinkedList)

    func swapTwoFirst() -> LinkedList {

        switch self {
        case .node(let value, .node(let nextValue, let next)):
            return .node(nextValue, .node(value, next.swapTwoFirst()))
        case .empty:
            return .empty
        case .node(let value, .empty):
            return .node(value, .empty)
        }
    }
}