Substitute methods

    public int substitute(int letter) {
        if((letter - position) >= 0){
            return this.mapping[letter-position] + position;
        }
        else{
            return this.mapping[((letter-position) + 26)] + position;
        }
    }

    public int substituteBack(int letter){
        if((letter - position) >= 0){
            return inverseMapping[letter - position] + position;
        }
        else{
            return inverseMapping[((letter - position) + 26)] + position;
        }
    }