题解 | #计算表达式#

#include <iostream>
#include <cstring>
#include <stack>
using namespace std;
bool isdigit(char x){
    if(x>='0' && x<='9') return true;
    else return false;
}
double getnum(string x,int &index){
    double k=0;
    while(isdigit(x[index])){
        k=k*10+x[index]-'0';
        index++;
    }
    return k;
    }
int priority(char x){
    if(x=='+'||x=='-') return 3;
    else if(x=='*'||x=='/') return 4;
    else if(x=='$') return 2;
    else return 1;
}
int main() {
    string exp;
    while(cin>>exp){
        stack<char> oper;
        stack<double> operand;
        oper.push('#');
        exp+='$';
        int i=0;
        while(i<exp.length()){
            if(isdigit(exp[i])) {
                double n=getnum(exp,i);
                operand.push(n);
            }
            else if( priority(exp[i])>priority(oper.top())) {
                oper.push(exp[i]);
                i++;
                }
            else {
                double o2=operand.top();
                operand.pop();
                double o1=operand.top();
                operand.pop();
                char op=oper.top();
                double r;
                if(op=='+') r=o1+o2;
                else if(op=='-') r=o1-o2;
                else if(op=='*') r=o1*o2;
                else if(op=='/') r=o1/o2;
                oper.pop();
                operand.push(r);
            }
            }
          cout<<operand.top()<<endl;  
        }
    }