// Pasos:
0: Borrar Espacios en blanco (trim)
1: IsNumber(string)
2: IsAddition(string)
3: IsSubstraction(string)
4: IsMultiplication(string)
5: IsDivision(string)

"1"
Number: 1

------------------------------------
"1 + 2"

Addition:
L: 1  (Number)
R: 2  (Number)

------------------------------------
"1 + 2 * 3"

Addition:
L: 1 (Number)
R: 2 * 3 (Multiplication)

Multiplication
L: 2 (Number)
R: 3 (Number)
V: 6

------------------------------------
"1 + 2 * 3 - 4"

Addition:
L: 1 (Number)
R: 2 * 3 - 4 (Substraction) (2)
V: 3
Substraction
L: 2 * 3 (Multiplication) (6)
R: 4 (Number) (4)
V: 2

Multiplication:
L: 2 (Number)
R: 3 (Number)
V: 6

------------------------------------
"2 + 2 * 3 - 4/2"
"2+2*3-4/2"

Addition:
L: 2 (Number)
R: 2*3-4/2 (Substraction) (4)
V: 6

Substraction:
L: 2*3 (Multiplication) (6)
R: 4/2 (Division) (2)
V: 4

Multiplication
L: 2 (Number)
R: 3 (Number)
V: 6

Division
L: 4 (Number)
R: 2 (Number)
V: 2

//pseudo codigo
Expression e = Expression.fromString("2 + 2 * 3 - 4/2");

//2+2*3-4/2
split("2+2*3-4/2", '+') => ["2", "2*3-4/2"];
split("2*3-4/2", '-') => ["2*3", "4/2"];
split("4/2", '/') => ["4", "2"];

expression = "2 + 2 * 3 - 4/2"
Expression* static fromString(string expression) {
    expression = trim(expression); //2+2*3-4/2

    if(IsNumber(expression)) {
        //detectar hasta donde llega el numero
        //parse string to number
        int i = itoa(expression);
        return new Number(i);
    } else if(IsAddition(expression)) {
        //partir string en operando izquierdo y derecho
        string operandos [] = split(expression, '+'); // ["2", "2*3-4/2"]
        string izquierdo = operandos[0];
        string derecho = operandos[1];

        return Addition(Expression.fromString(izquierdo), Expression.fromString(derecho));
    } else if (IsSubstraction(expression)) {
        //partir string en operando izquierdo y derecho
        // expression = 2*3-4/2
        string operandos [] = split(expression, '-'); // ["2*3", "4/2"]
        string izquierdo = operandos[0];
        string derecho = operandos[1];

        return Substraction(Expression.fromString(izquierdo), Expression.fromString(derecho));
    } else if (IsMultiplication(expression)) {
        //partir string en operando izquierdo y derecho
        // expression = 2*3
        string operandos [] = split(expression, '*'); // ["2", "3"]
        string izquierdo = operandos[0];
        string derecho = operandos[1];

        return Multiplication(Expression.fromString(izquierdo), Expression.fromString(derecho));

    } else if (IsDivision(expression)) {
        //partir string en operando izquierdo y derecho
        // expression = 4/2
        string operandos [] = split(expression, '/'); // ["4", "2"]
        string izquierdo = operandos[0];
        string derecho = operandos[1];

        return Multiplication(Expression.fromString(izquierdo), Expression.fromString(derecho));
    }
}