栈的应用

#include<iostream>
#include<cstring>
#include<stack>
using namespace std;

stack<int> s1;
stack<char> s2;

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

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

int main(){
    string s;
    cin >> s;
    for(int i = 0; i < s.size(); i ++){
        char c = s[i];
        if(c >= '0' && c <= '9'){
            int res = c - '0';
            while(i + 1 < s.size() && s[i + 1] >= '0' && s[i + 1] <= '9'){
                res = res * 10 + (s[i + 1] - '0');
                i ++;
            }
            s1.push(res);
        }
        else{
            if(c == '(') s2.push(c);
            else if(c == ')'){
                while(s2.top() != '('){
                    int a = s1.top();
                    s1.pop();
                    int b = s1.top();
                    s1.pop();
                    char op = s2.top();
                    s2.pop();
                    s1.push(get_res(a, b, op));
                }
                s2.pop();
            }
            else{
                while(!s2.empty() && get_pri(c) <= get_pri(s2.top()) ){
                    int a = s1.top();
                    s1.pop();
                    int b = s1.top();
                    s1.pop();
                    char op = s2.top();
                    s2.pop();
                    s1.push(get_res(a, b, op));
                }
                s2.push(c);
            }
        }
    }
    while(!s2.empty()){
        int a = s1.top();
        s1.pop();
        int b = s1.top();
        s1.pop();
        char op = s2.top();
        s2.pop();
        s1.push(get_res(a, b, op));
    }
    cout << s1.top() << endl;
    return 0;
}