gistfile1.swift

extension String {
    var isHomogeneous: Bool {
        guard !isEmpty else { return true }

        for i in startIndex..<endIndex.predecessor() {
            if characters[i] != characters[i.successor()] {
                return false
            }
        }
        return true
    }
}

assert("aaa".isHomogeneous)
assert("😎😎".isHomogeneous)
assert("😎".isHomogeneous)
assert("".isHomogeneous)

assert("as".isHomogeneous == false)
assert(" </del>~".isHomogeneous == false)