数据结构--表达式求值

https://www.acwing.com/problem/content/3305/

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