Determine whether given string of parentheses is properly nested in Swift

nested.swift

func isNestedString(_ S : String) -> Bool {
    if S.isEmpty {
        return true
    }
    
    var stack: [Character] = [Character]()
    
    for elem in S {
        if elem == "(" {
            stack.append(elem)
        }else if elem == ")" {
            guard let _ = stack.popLast() else { return false }
        }
    }
    
    return stack.isEmpty
}