表达式求值

#include<iostream>
#include<sstream>
#include<string>
#include<cstdio>
typedef long long LL;
using namespace std;

stringstream ss;


LL factorValue();       //计算一个因子值 (因子由 整数 或 (表达式)构成)
LL termValue();         //计算一个项的值
LL expressionValue();       //计算一个表达式值


//计算一个表达式值
LL expressionValue(){
    LL result = termValue();
    bool more = true;
    while(more){
        char c = ss.peek();
        if(c == '+'){
            ss.get();
            result += termValue();
        }
        else if(c == '-'){
            ss.get();
            result -= termValue();
        }
        else
            more = false;
    }
    return result;
}

LL termValue(){
    LL result = factorValue();
    bool more = true;
    while(more){
        char c = ss.peek();
        if(c == '*' || c == '/'){
            ss.get();
            result = c == '*' ? result * factorValue() : result / factorValue();
        }
        else more = false;
    }

    return result;
}

LL factorValue(){
    LL result = 0;
    char c = ss.peek();
    if(c == '('){
        ss.get();
        result = expressionValue();
        ss.get();       //拿掉右括号
    }
    else{
        //读取一个整数
        int num;
        while(ss.peek() >= '0' && ss.peek() <= '9'){
            num = ss.get() - '0';
            result = result * 10 + num;
        }
    }
    return result;
}
int main(){
    //io流换成string流是为了模拟题目要求直接处理给定字符串情况。
    string s;
    cin >> s;
    ss << s;
    cout << expressionValue() << endl;

    return 0;
}
//(1+2)*2+45*2