题解 | #表达式求值#

#include<iostream>
#include<stack>
using namespace std;
int pos;
int result(string &data)
{
    int num=0;
    char flag = '+';
    stack<int> st;
    int len = data.length();
    while(pos<len)
    {
        if(data[pos] == '(')
        {
            pos++;
            num = result(data);
        }
        while(pos<len && data[pos]-'0'>=0 && data[pos]-'9'<=0)
        {
            num =num*10+ data[pos] - '0';
            pos ++;
        }
        switch(flag)
        {
            case '+':
            {
                st.push(num);
                break;
            }
            case '-':
            {
                st.push(-num);
                break;
            }
            case '*':
            {
                int tmp = st.top();
                st.pop();
                tmp *= num;
                st.push(tmp);
                break;
            }
            case '/':
            {
                int tmp = st.top();
                st.pop();
                tmp /= num;
                st.push(tmp);
                break;
            }
        }
        num = 0;
        flag = data[pos];
        if(data[pos] == ')')
        {
            pos ++;
            break;
        }
        pos ++;
    }
    int sum = 0;
    while(st.size())
    {
        sum += st.top();
        st.pop();
    }
    return sum;
}
int main()
{
    string str;
    while(cin>>str)
    {
        pos = 0;
        cout<<result(str)<<endl;
    }
    return 0;
}