表达式求值

/*
    表达式求值，逆波兰式(后缀表达式)算法
    输入(可以有空格，支持小数，实现‘+-/*‘)： ((1+2)*5+1)/4=
    转换为后缀表达式： 得到：1 2 + 5 * 1 + 4 / =
    计算后缀表达式：得到：4.00
*/
struct Exp  {
    stack<char> op;
    stack<double> num;
    bool error;

    int prior(char ch)	{                           //运算符的优先级
    	switch (ch)	{
    		case ‘+‘:
    		case ‘-‘: return 1;
    		case ‘*‘:
    		case ‘/‘: return 2;
    		default:  return 0;
    	}
    }
    bool is_digit(char ch)	{
    	return ‘0‘ <= ch && ch <= ‘9‘;
    }
    string get_postfix(string s)    {               //中缀表达式转变后缀表达式 
        while (!op.empty ())    op.pop ();
        op.push (‘#‘);
        string ret = "";
        int len = s.length (), i = 0;
        while (i < len)    {
            if (s[i] == ‘ ‘ || s[i] == ‘=‘)    {
                i++;    continue;
            }
            else if (s[i] == ‘(‘)    {
            	op.push (s[i++]);
            }
            else if (s[i] == ‘)‘)	{
            	while (op.top () != ‘(‘)	{
            		ret += op.top ();	ret += ‘ ‘;
            		op.pop ();
            	}
            	op.pop ();	i++;
            }
            else if (s[i] == ‘+‘ || s[i] == ‘-‘ || s[i] == ‘*‘ || s[i] == ‘/‘)	{
            	while (prior (op.top ()) >= prior (s[i]))	{
            		ret += op.top ();	ret += ‘ ‘;
            		op.pop ();
            	}
            	op.push (s[i++]);
            }
            else	{
            	while (is_digit (s[i]) || s[i] == ‘.‘)	{
            		ret += s[i++];
            	}
            	ret += ‘ ‘;
            }
        }
        while (op.top () != ‘#‘) {
            ret += op.top ();	ret += ‘ ‘;
            op.pop ();
        }
        ret += ‘=‘;
        return ret;
    }
    double cal(double a, double b, char ch)	{
    	if (ch == ‘+‘)	return a + b;
    	if (ch == ‘-‘)	return a - b;
    	if (ch == ‘*‘)	return a * b;
    	if (ch == ‘/‘)	{
    		if (b != 0)	return a / b;
    		error = true;	return 0;
    	}
    }
    double solve(string str)	{                   //计算后缀表达式
    	string s = get_postfix (str);
    	while (!num.empty ())	num.pop ();
        error = false;
    	int len = s.length (), i = 0;
    	while (i < len)	{
    		if (s[i] == ‘ ‘ || s[i] == ‘=‘)	{i++;	continue;}
    		else if (s[i] == ‘+‘ || s[i] == ‘-‘ || s[i] == ‘*‘ || s[i] == ‘/‘)	{
    			double a = num.top ();	num.pop ();
    			double b = num.top ();	num.pop ();
    			num.push (cal (b, a, s[i]));	i++;
    		}
    		else  {
    			double x = 0;
    			while (is_digit (s[i]))	{
    				x = x * 10 + s[i] - ‘0‘;	i++;
    			}
    			if (s[i] == ‘.‘)	{
    				double k = 10.0, y = 0;
   					i++;
   					while (is_digit (s[i]))	{
   						y += ((s[i] - ‘0‘) / k);
                        i++;    k *= 10;
   					}
   					x += y; 
    			}
                num.push (x);
    		}
    	}
    	return num.top ();
    }
}E;

int main(void)    {
    // ios::sync_with_stdio (false);　　　　　　　　　　　//如果全用流的话，加这句话能跑快点
	int T;	scanf ("%d", &T);   getchar ();
    string str;
	while (T--)	{
		getline (cin, str);
        cout << E.get_postfix (str) << endl;
		printf ("%.2f\n", E.solve (str));
	}

    return 0;
}