Reverse Polish Notation

/*
 *hieudoan7 18:19 02/07/2019
 */
#include <bits/stdc++.h>
using namespace std;

int getPriority(char c){
    if(c == '*' || c=='/') return 2;
    else if (c=='+' || c=='-') return 1;
    else if (c=='(' || c==')') return 0;
    else return -1; //number
}

string toRPN(string s){
    stack<char> st;
    string ans = "";
    for(int i=0; i<s.length(); i++){
        int prior = getPriority(s[i]);
        if(prior == -1) ans += s[i];
        else { //operator
            if(s[i]=='(') st.push(s[i]);
            else {
                if(s[i]==')') {
                    while(st.top()!='(') {
                        ans+=st.top();
                        st.pop();
                    }
                    st.pop(); //pop (
                }
                else {
                    while(!st.empty() && prior <= getPriority(st.top())){
                        ans += st.top();
                        st.pop();
                    }
                    st.push(s[i]);
                }
            }
        }
    }
    while(!st.empty()) {
        ans += st.top();
        st.pop();
    }
    return ans;
}

int aOpeb(int a, int b, char ope){
    if(ope == '*') return a*b;
    if(ope == '/') return a/b;
    if(ope == '+') return a+b;
    if(ope == '-') return a-b;
 }

int calculate (string RPN){
    int ans = 0;
    stack<int> st;
    for(int i=0; i<RPN.length(); i++){
        int prior = getPriority(RPN[i]);
        if(prior == -1) st.push(RPN[i]-'0'); //mọi toán hạng đều có 1 chữ số ;v
        else {
            int b = st.top(); st.pop();
            int a = st.top(); st.pop();
            st.push(aOpeb(a,b,RPN[i]));
        }
    }
    return st.top();
}

int main()
{
    string S = "3+5*(6-3)+1";
    string RPN = toRPN(S);
    cout << RPN <<endl;
    cout << S << " = " << calculate(RPN) <<endl;

    return 0;
}