Infix to Postfix

Parser.cc

#include <iostream>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>

using namespace std;

unordered_map<char, unsigned> const prec = {
    { '^', 1 },
    { '*', 2 },
    { '/', 2 },
    { '+', 3 },
    { '-', 3 }
};

string Parse(char const* str)
{
    vector<std::pair<char, unsigned>> stack;
    string rtn;
    while (*str)
    {
        char c = *(str++);
        if (auto it = prec.find(c); it != prec.end())
        {
            if (stack.empty() || stack.back().second > it->second)
                stack.push_back(*it);
            else
            {
                while (!stack.empty()
                       && stack.back().first != '('
                       && stack.back().second <= it->second)
                {
                    rtn.push_back(stack.back().first);
                    stack.pop_back();
                }
                stack.push_back(*it);
            }
        }
        else if (c == '(')
            stack.push_back({ c, 0 });
        else if (c == ')')
        {
            if (stack.empty())
                return "PARSE ERROR";

            while (stack.back().first != '(')
            {
                rtn.push_back(stack.back().first);
                stack.pop_back();
            }
            stack.pop_back();
        }
        else
            rtn.push_back(c);
    }
    for (auto it = stack.rbegin(); it != stack.rend(); ++it)
        rtn.push_back(it->first);
    return rtn;
}

int main(int argc, char* argv[])
{
    for (int i = 1; i < argc; ++i)
        cout << Parse(argv[i]) << endl;
    return 0;
}