func swapCharacters(s: String, index1: Int, index2: Int) -> String {
var characters = Array(s)
characters.swapAt(index1, index2)
return String(characters)
}
let s = "Swift programming language"
print(s);
print(swapCharacters(s: s, index1: 4, index2: 9))
/*
run:
Swift programming language
Swifg protramming language
*/