countPairs.swift

func countPairs(numbers: [Int], k: Int) -> Int {
    struct Pair: Equatable, Hashable {
        let lhs: Int
        let rhs: Int
        
        func isValid(k: Int) -> Bool {
            return lhs + k == rhs
        }
        
        static func ==(lhs: Pair, rhs: Pair) -> Bool {
            return rhs.contains(element: lhs.lhs) || rhs.contains(element: lhs.rhs)
        }
        
        private func contains(element: Int) -> Bool {
            return lhs == element || rhs == element
        }
    }
    
    struct ValidPairsContainer {
        private var pairs: Set<Pair> = []
        
        var count: Int {
            return pairs.count
        }
        
        mutating func appendIfValid(lhs: Int, rhs: Int, k: Int) {
            let pair = Pair(lhs: lhs, rhs: rhs)
            guard pair.isValid(k: k) else { return }
            pairs.insert(pair)
        }
    }
            
    var container = ValidPairsContainer()
    
    let numbersSet = Set(numbers)
    
    numbersSet.forEach { lhsNumber in
        let rhsNumber = lhsNumber + k
        guard numbersSet.contains(rhsNumber) else { return }
        container.appendIfValid(lhs: lhsNumber, rhs: rhsNumber, k: k)
    }
    
    return container.count
}