shunting yard

#include <bits/stdc++.h>

using namespace std;

int main()
{
    vector<char>output;
    list<char>stack;
    list<char>::iterator it = stack.begin();

    string expr("2+2*3+3+5");

    map<char,int> op_precedence;

    op_precedence['+'] = 10;
    op_precedence['-'] = 10;
    op_precedence['*'] = 20;
    op_precedence['/'] = 20;

    for(char &c : expr){

        if(isdigit(c)){
            output.push_back(c);
        }else{

            if(stack.size() > 0){

                if(op_precedence[stack.front()] >= op_precedence[c]){
                    output.push_back(stack.front());
                    stack.pop_front();
                }
            }
            stack.push_back(c);
        }
    }

    for(it = stack.begin(); it!= stack.end(); it++){
        output.push_back(*it);
    }

    for(auto &i : output){
        cout<<i<<' ';
    }
    return 0;
}