This is the code I wrote for the problem "Expression Tree" on Judgegirl. Here is the link to the question, and I also save the problem description in 65.md for easier access. I'll gradually refactor the code using the techniques taught in class (CSIE1211).

I can use a stack to store the prefix expression and evaluate it when needed. Luckily, the given expressions have parenthesis, making them easy to parse. I can read the expression from left to right and if I see a (, I can just ignore it. If I see a ), I know I can take out three items (two operands and one operator) to evaluate then push the result to stack. Otherwise, simply push the item to the stack. In the end, the only remaining item in the stack is the answer.

There are three classes, Item, VariableSet, and Statements.

Item is for the item stored in the stack, whether it be an integer, operator, or variable. I also implement arithmetic in it, since I think it's like an operator takes two operands to operate.

VariableSet is for storing the name and value of the variables, and a method to find them.

Statements is for storing and running the expression. Since the input is separated by spaces, I use stringstream to separate them.

To check whether my code is correct, I wrote a shell script to download test data and another script to compare the output of my code with the answer. I can run ./get 65 to download test data for this problem and ./test to test the code.