中缀表达式转换为后缀(只能是字母)

#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define ll long long
using namespace std;

vector<char> st;

int main() {
    string s;
    cin>>s;
    map<char,ll> mp;
    mp['+'] = mp['-']=0;
    mp['*'] = mp['/']=1;
    mp['('] = mp[')']=2;
    for(int i =0;i<s.size();i++){
        if(s[i]<='z' && s[i]>= 'a')
            cout<<s[i];
        else{
            if(s[i]    == ')'){
                while(st.back()!='('){    
                    cout<<st.back();
                    st.pop_back();
                }
                st.pop_back();
            }
            else{                
                while(!st.empty() && mp[st.back()]>=mp[s[i]] && st.back()!='('){
                    cout<<st.back();
                    st.pop_back();
                }
                st.push_back(s[i]);
            }
        }
    }
    while(!st.empty()){
        cout<<st.back();
        st.pop_back();
    }
    return 0;
}
//a+b*c+(d*e+f)*g