For the reddit 135 easy challenge, uses rpn (probably badly implemented), very messy code, I will make a calculator using this method and try to write cleaner code and understand the whole thing better....

reddit_135_easy

#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <stack>
#include <vector>
#include <queue>
#include <deque>
#include <string>
#include <sstream>
#include <iostream>

using namespace std;

bool IsOp(char token, vector<char>& op)
{
    for(unsigned int i = 0; i < op.size(); ++i)
    {
        if(token == op[i])
            return true;
    }

  return false;
}

bool IsOp(string token, vector<char>& op)
{
    for(unsigned int i = 0; i < op.size(); ++i)
    {
        string tmp(1, op[i]);
        if(tmp == token) return true;
     }
    return false;
}

double String2Number(string &token)
{
    istringstream ss(token);
    double num;

    ss >> num;
    return num;
}



void ParseString(string& input, queue<string>& output, vector<char>& vec)
{

    stack<string> op_stack;

    string number;
    bool presedence = false;
    bool in_digit = false;

    for(unsigned int i = 0; i < input.length(); ++i)
    {
        if(isdigit(input[i]))
        {
            in_digit = true;
            number += input[i];
        }
        if(input[i] == ' ' || i == input.length()-1)
        {
            if(in_digit == true)
            {
              output.push(number);
              if(presedence == true)
              {
                  string op = op_stack.top();
                  op_stack.pop();
                  output.push(op);
                  presedence = false;
              }
              in_digit = false;
              number.clear();
            }
        }
        if(IsOp(input[i], vec))
        {
            string tmp(1, input[i]);
            op_stack.push(tmp);

            if(input[i] == '*')
            {
                presedence = true;

            }
        }
    }

    stack<string> tmp;

    while(!op_stack.empty())
    {
        string op = op_stack.top();
        op_stack.pop();
        tmp.push(op);
    }

    while(!tmp.empty())
    {
      string op = tmp.top();
      tmp.pop();
      output.push(op);
    }

}


double Calculate(queue<string>& output, vector<char> & op)
{
    deque <double> numbers;

    while(!output.empty())
    {
        string tmp = output.front();
        output.pop();

        if(IsOp(tmp, op))
        {
           int m, l, r = 0;

           if(tmp == "*")
           {
               m = numbers.back();
               numbers.pop_back();
               l = numbers.back();
               numbers.pop_back();
           }
           else
           {
               m = numbers.front();
               numbers.pop_front();
               l = numbers.front();
               numbers.pop_front();
           }

           if(tmp == "+") r = m + l;
           if(tmp == "-") r = m - l;
           if(tmp == "*") r = m * l;

           if(tmp == "*")
               numbers.push_back(r);
           else
               numbers.push_front(r);
        }
        else
        {
            numbers.push_back(String2Number(tmp));
        }
    }


    double result = numbers.front();
    return result;

}

string Int2String(int t)
{
    stringstream ss;
    ss << t;
    return ss.str();
}

int main()
{
    queue<string> out_queue;

    vector<char> operators;
    operators.push_back('+');
    operators.push_back('-');
    operators.push_back('*');

    srand(time(0));

    int M, N;
    cin>>M>>N;

    while(1)
    {
    vector<string> r_num;
    for(int i = 0; i < 4; i++)
    {
        int tmp = (rand() % N + M);
        r_num.push_back(Int2String(tmp));
    }

    vector<char> r_ops;

    for(int i = 0; i < 3; i++)
      r_ops.push_back(operators[rand()%3]);

    string expression = r_num[0] + " " + r_ops[0] + " " + r_num[1] + " " + r_ops[1] + " " + r_num[2] \
            +" " + r_ops[2] + " " + r_num[3];

    cout<<"> "<<expression<<endl;

    ParseString(expression, out_queue, operators);

   int comp_result = Calculate(out_queue, operators);
   int i_result = 0;

   string s_result;
   cin>>s_result;

   if(s_result == "q")
       return 0;


   i_result = String2Number(s_result);


   if(i_result == comp_result)
       cout<<"Correct!"<<endl;

    }

    return 0;
}