题解 | #四则运算#

#include<iostream>
#include<sstream>
#include<stack>

using namespace std;

void inStack(stack<double>& st, char ch, double value){
    if(ch=='+'){
        st.push(value);
    }
    else if(ch == '-'){
        st.push(-value);
    }
    else if(ch == '*'){
        double a=st.top();
        st.pop();
        st.push(a*value);
    }
    else{
        double a=st.top();
        st.pop();
        st.push(a/value);
    }
}
double calculate(string s){
    stack<double> st;
    st.push(0);
    char ch = '+';
    stack<char> st_ch;
    int i=0, len = s.length();
    while(i<len){
        if(isdigit(s[i])){
            double num = 0;
            int j=i;
            while(j<len&&isdigit(s[j])){
                num = num*10+(s[j]-'0');
                j++;
            }
            i=j-1;
            inStack(st, ch, num);
        }
        else if(s[i]=='{'||s[i]=='['||s[i]=='('){
            st_ch.push('(');
            int j=i;
            while(j<len&&st_ch.size()!=0){
                j++;
                if(s[j]==')'||s[j]=='}'||s[j]==']'){
                    st_ch.pop();
                }
                else if(s[j]=='{'||s[j]=='['||s[j]=='('){
                    st_ch.push('(');
                }
            }
            string temp = s.substr(i+1, j-i-1);
            double value = calculate(temp);
            inStack(st, ch, value);
            i = j;
        }
        else{
            ch = s[i];
        }
        i++;
    }
    double ans = 0;
    while(!st.empty()){
        ans+=st.top();
        st.pop();
    }
    return ans;

}
int main(){
    string s;
    while(cin>>s){
        double ans = calculate(s);
        cout<<ans<<endl;
    }
}