求混合加减乘除的答案

#include<bits/stdc++.h>
using namespace std;

stack<int>num;
stack<char>op;

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