Pairs of Sum K.swift

extension Sequence where Element == Int {
    func pairsOfSum(k: Int) -> [(Int, Int)] {
        var cache = [Int:Int]()
        var result = [(Int, Int)]()
        
        for i in self {
            cache[i] = k - i
        }
        
        for (num, inv) in cache {
            if cache[inv] != nil {
                result.append((num, inv))
            }
        }
        
        return result
    }
}