题解 | #简单计算器#

//高优先级压栈，低或相等优先级弹栈计算即可
#include "stdio.h"
#include "string"
#include "stack"
#include "map"
using namespace std;
stack<char> operStack;
stack<double> numStack;
string str;

void calculate(){
    char operation = operStack.top();operStack.pop();
    double a = numStack.top();numStack.pop();
    double b = numStack.top();numStack.pop();
    double num;
    switch (operation) {
        case '+':
            num=a+b;
            numStack.push(num);break;
        case '-':
            num=b-a;
            numStack.push(num);break;
        case '*':
            num=a*b;
            numStack.push(num);break;
        case '/':
            num=b/a;
            numStack.push(num);break;
        default:
            break;
    }
}

int main(){
    char buf[300];
    map<char,int> priority = {
            {'$',0},
            {'+',1},{'-',1},
            {'*',2},{'/',2}
    };
    while (fgets(buf,300,stdin)!=NULL){
        str=buf;
        str.pop_back();
        if(str=="0")
            break;
        str.push_back('$');
        string num;
        for (int i = 0; i < str.size(); ++i) {
            if(str[i]>='0' && str[i]<='9'){ //是数字
                num.push_back(str[i]);
            } else if(str[i] == ' '){//为空格时
                if(!num.empty()){ //若数字不为空，则压入栈中
                    numStack.push(stod(num));
                    num.clear();
                }
            } else{ //此为+ - * /情况
                if(str[i]=='$'){
                    if(!num.empty()){ //若数字不为空，则压入栈中
                        numStack.push(stod(num));
                        num.clear();
                    }
                }
                while (!operStack.empty() && priority[operStack.top()] >= priority[str[i]]){//要弹栈
                    calculate();
                }
                operStack.push(str[i]);
            }
        }
        printf("%.2lf\n",numStack.top());
        numStack.pop();
        str.clear();
    }
}