题解 | #简单计算器#

#include <bits/stdc++.h>
#include <cstddef>
#include <cstdio>
#include <stack>
#include <string>
using namespace std;

int main() {
    char buf[1001];
    while (fgets(buf, 1001, stdin) != NULL) {
        string expr = buf;
        expr.pop_back();
        map<char, int> pirority{
            {'$', 0},
            {'+', 1}, {'-', 1},
            {'*', 2}, {'/', 2},
        };
        //去除换行符号
        /*  整数和运算符之间用一个空格分隔。没有非法表达式。当一行中只有0时输入结束，相应的结果不要输出。
                    输入：
                    1 + 2
                    4 + 2 * 5 - 7 / 11
                    0
                    输出：
                    3.00
                    13.36
                    */
        if (expr == "0") break;
        expr.push_back('$');//补充一个虚拟的终止符号
        string num;//数字
        stack<double> numstack;
        stack<char> operstack;
        for (unsigned i = 0; i < expr.size(); i++) {
            if (expr[i] >= '0' && expr[i] <= '9') {
                num.push_back(expr[i]);//压入数字
            } else if (expr[i] == ' ') {
                if (num != " ") {
                    numstack.push(stod(num));//stod():string->double
                    num = " ";
                }
            } else {
                if (expr[i] == '$') {
                    if (num != " ") {
                        numstack.push(stod(num));
                        num = " ";
                    }
                }
                while (!operstack.empty() && pirority[operstack.top()] >= pirority[expr[i]]) {
                    char oper = operstack.top();
                    operstack.pop();
                    double right = numstack.top();
                    numstack.pop();
                    double left = numstack.top();
                    numstack.pop();
                    switch (oper) {
                        case '+':
                            numstack.push(right + left);
                            break;
                        case '-':
                            numstack.push(left - right);
                            break;
                        case '*':
                            numstack.push(right * left);
                            break;
                        case '/':
                            numstack.push(left /right );
                            break;
                    }
                }
                operstack.push(expr[i]);
            }
        }

    
    printf("%.2lf\n",numstack.top());
}
}
// 64 位输出请用 printf("%lld")