String+Pyramid.swift

extension String {
    func pyramid() -> String {
        var result = ""
        for index in 1..<characters.count {
            result.append(contentsOf: self.characters.prefix(index))
            result.append("\n")
        }
        for index in stride(from: characters.count, to: 0, by: -1) {
            result.append(contentsOf: self.characters.prefix(index))
            result.append("\n")
        }
        return result
    }
}