C++简易计算器：包括加减乘除和括号

输入：任意包括'+','-','*','/','(',')'空格以及数字（可以是浮点数）的合法运算表达式
输出：运算结果（浮点数）
代码实现：

#include <bits/stdc++.h>
using namespace std;
class Solution {
    public:
    double calculate(string s){
        // initialize
        vector<string> init;
        int len = s.length();
        string mem = "";
        bool minus_check = false;
        for(int i=0; i<len; ++i){
            switch(s[i]){
                case '(': case ')': case '+': case '-': case '*': case '/':
                    if(mem.length()){
                        init.push_back(mem);
                        mem = "";
                        minus_check = true;
                    }
                    if(s[i] == '-' && minus_check == false){
                        init.push_back("0");
                    }
                    mem += s[i];
                    init.push_back(mem);
                    mem = "";
                    if (s[i] == '(')
                        minus_check = false;
                break;
                case ' ':
                    if(mem.length()){
                        init.push_back(mem);
                        mem = "";
                        minus_check = true;
                    }
                break;
                default:
                    mem += s[i];
                break;
            }
        }
        if(mem.length()){
            init.push_back(mem);
            mem = "";
        }
        // process
        stack<string> st1;
        deque<string> st2;
        int siz = init.size();
        for(int i=siz-1; i>=0; --i){
            if(init[i] != "("){
                st1.push(init[i]);
            }
            if(init[i] == "(" || i == 0){
                while(!st1.empty() && st1.top() != ")"){
                    string opn = st1.top();
                    st1.pop();
                    if(opn == "*" || opn == "/"){
                        double rn = stod(st1.top());
                        st1.pop();
                        double ln = stod(st2.back());
                        st2.pop_back();
                        if(opn == "*") ln *= rn;
                        else ln /= rn;
                        st2.push_back(to_string(ln));
                    }
                    else{
                        st2.push_back(opn);
                    }
                }
                if(!st1.empty()) st1.pop();
                double ans = 0;
                while(!st2.empty()){
                    string opn = st2.front();
                    st2.pop_front();
                    if(opn == "+" || opn == "-"){
                        double rn = stod(st2.front());
                        st2.pop_front();
                        if(opn == "+") ans += rn;
                        else ans -= rn;
                    }
                    else{
                        ans = stod(opn);
                    }
                }
                st1.push(to_string(ans));
            }
        }
        double ans = 0;
        while(!st1.empty()){
            string opn = st1.top();
            st1.pop();
            if(opn == "+" || opn == "-"){
                double rn = stod(st1.top());
                st1.pop();
                if(opn == "+") ans += rn;
                else ans -= rn;
            }
            else{
                ans = stod(opn);
            }
        }
        return ans;
    }
};
int main(){
    string cals;
    cin>>cals;
    Solution sol;
    double ans = sol.calculate(cals);
    cout<<ans<<endl;
}