[Swift]String extension for competitive programming

String+Competitive.swift

//Apple Swift version 2.1.1 (swiftlang-700.1.101.15 clang-700.1.81)
extension String: CollectionType {
    func at(idx: Int) -> Character {
        return self[self.startIndex.advancedBy(idx)]
    }
    func subString(s: Int, _ e: Int) -> String {
        return self[self.startIndex.advancedBy(s)..<self.startIndex.advancedBy(e)]
    }
}

"abcdef".at(1) //"b": Character
"012345".count //6
"012345".subString(2, 5) //"234"
"abc3defg7".split { "0"..."9" ~= $0 } //["abc", "defg"]
"4 5a 5 a 7 ".split(" ").flatMap { Int($0) } //[4, 5, 7]
["a", "b", "c"].joinWithSeparator("+") //"a+b+c"