acwing3302. 表达式求值

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

stack<int>num;
stack<char>op;
void eval(){
    auto b=num.top();num.pop();
    auto a=num.top();num.pop();
    auto c=op.top();op.pop();
    int x;
    if(c=='+')x=a+b;
    else if(c=='-')x=a-b;
    else if(c=='*')x=a*b;
    else x=a/b;
    num.push(x);
}
int main(){
    unordered_map<char,int>pr={{'+',1},{'-',1},{'*',2},{'/',2}};
    string s;
    cin>>s;
    for(int i=0;i<s.size();i++){
        auto c=s[i];
        if(isdigit(c)){
            int x=0,j=i;
            while(j<s.size()&&isdigit(s[j]))x=x*10+s[j++]-'0';
            i=j-1;
            num.push(x);
        }else if(s[i]=='(')op.push('(');
        else if(s[i]==')'){
            while(op.top()!='(')eval();
            op.pop();
        }else{
            while(op.size()&&op.top()!='('&&pr[op.top()]>=pr[c])eval();
            op.push(c);
        }
    }
    while(op.size())eval();
    cout<<num.top()<<endl;
    
    return 0;
}