Interpret

#include <iostream>
#include <map>
#include <stack>

using namespace std;

class Expression{
public:
    virtual int interpret(map<char, int>var) = 0;
    virtual ~Expression(){}
};

//变量表达式
class VarExpression : public Expression{
    char key;
public:
    VarExpression(const char & key){ this->key = key; }
    int interpret(map<char, int> var)override{
        return var[key];
    }

};

//符号表达式
class SymbolExpression : public Expression{
    //运算符左右有两个参数
protected:
    Expression * left;
    Expression * right
;public:
    SymbolExpression(Expression * left, Expression * right):left(left), right(right){}
};

//加法运算
class AddExpression : public SymbolExpression{
  public:

    AddExpression(Expression * left, Expression * right):SymbolExpression(left, right){}
    int interpret(map<char, int> var) override
    {
        return left->interpret(var) + right->interpret(var);
    }
};


//减法运算
class SubExpression : public SymbolExpression{
  public:

    SubExpression(Expression * left, Expression * right):SymbolExpression(left, right){}
    int interpret(map<char, int> var) override
    {
        return left->interpret(var) - right->interpret(var);
    }
};

Expression * analyse(string expStr){
    stack<Expression * > expStack;
    Expression * left = nullptr;
    Expression * right = nullptr;
    for(int i = 0; i < expStr.size(); i++){
        switch(expStr[i]){
        case '+':
            left = expStack.top();
            right = new VarExpression(expStr[++i]);
            expStack.push(new AddExpression(left, right));
            break;
        case '-':
            left = expStack.top();
            right = new VarExpression(expStr[++i]);
            expStack.push(new SubExpression(left, right));
            break;

        default:
            expStack.push(new VarExpression(expStr[i]));

        }
    }

    Expression * expression = expStack.top();
    return expression;
}



int main()
{
    cout << "Hello World!" << endl;

    string expStr = "a+b-c+d";
    map<char, int>var;
    var['a'] = 5;
    var['b'] = 2;
    var['c'] = 1;
    var['d'] = 6;

    Expression * expression = analyse(expStr);

    int result = expression->interpret(var);

    cout<<result<<endl;


    return 0;
}

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