题解 | #简单计算器#

#include <iostream>
#include <stack>
#include <map>
using namespace std;

double func(double x, double y, char c) {
    if (c == '+') return x + y;
    else if (c == '-') return x - y;
    else if (c == '*') return x * y;
    else if (c == '/') return x / y;
    return -1;
}
int main() {
    map<char, int> m;
    m['+'] = 1, m['-'] = 1, m['*'] = 2, m['/'] = 2;
    string str;
    while (getline(cin, str)) {
        if (str == "0") break;
        stack<double> nums;
        stack<char> op;
        for (int i = 0; i < str.size(); i++) {
            if (isdigit(str[i])) {
                int j = i;
                while (j < str.size() && isdigit(str[j])) j++;
                //i~j-1是一个数
                int t = 0;
                for (int k = i; k < j; k++) t = 10 * t + str[k] - '0';
                nums.push(t);
                i = j - 1; //i++以后指向下一个字符
            } else if (str[i] != ' ') { //读到了运算符
                while (!op.empty() && m[op.top()] >= m[str[i]]) {
                    double y = nums.top(); //右操作数
                    nums.pop();
                    double x = nums.top(); //左操作数
                    nums.pop();
                    char c = op.top();
                    op.pop();
                    double res = func(x, y, c);
                    nums.push(res);
                }
                op.push(str[i]);
            }
        }
        while (!op.empty()) {
            double y = nums.top(); //右操作数
            nums.pop();
            double x = nums.top(); //左操作数
            nums.pop();
            char c = op.top();
            op.pop();
            double res = func(x, y, c);
            nums.push(res);
        }
        printf("%.2lf\n",nums.top());
        nums.pop();
    }
    return 0;
}