表达式求值

有人问为什么一般转后缀表达式求值不转前缀，因为后缀求值的时候可以递推，前缀求值的时候要递归，递推效率显然比递归效率高

#include <iostream>
#include <stack>
using namespace std;

double toNum(char*s, int &k)
{
	int flag = 0;
	double x = 0.0, y = 0.1;
	while (s[k] >= '0'&&s[k] <= '9' || s[k] == '.')
	{
		if (s[k] >= '0'&&s[k] <= '9')
		{
			if (flag == 0)
				x = x * 10 + s[k] - '0';
			else
			{
				x = x + y*(s[k] - '0');
				y = y*0.1;
			}
		}
		else
		{
			flag = 1;
		}
		k = k + 1;
	}
	return x;
}

int priority(char c)
{
	int k;
	switch (c)
	{
	case '*':k = 2; break;
	case '/':k = 2; break;
	case '+':k = 1; break;
	case '-':k = 1; break;
	case '(':k = 0; break;
	case ')':k = 0; break;
	default:k = -1; break;
	}
	return k;
}

int main()
{
	stack<double> sv;
	stack<char> sp;
	char c;
	int k = 0, flag = 1;
	double x, y;
	sp.push('\0');
	char *s = new char[100];
	cout << "Input:" << endl;
	cin >> s;
	c = s[k];
	while (flag)
	{
		if (c >= '0'&&c <= '9' || c == '.')
		{
			sv.push(toNum(s, k));
		}
		else if (c == '\0'&& sp.top() == '\0')
		{
			flag = 0;
		}
		else if (c == '(' || (priority(c) > priority(sp.top())))
		{
			sp.push(c);
			k++;
		}
		else if (c == ')'&& sp.top() == '(')
		{
			sp.pop();
			k++;
		}
		else if (priority(c) <= priority(sp.top()))
		{
			x = sv.top();
			sv.pop();
			y = sv.top();
			sv.pop();
			c = sp.top();
			sp.pop();
			switch (c)
			{
			case '+':y = x + y; break;
			case '-':y = y - x; break;
			case '*':y = x*y; break;
			case '/':y = y / x; break;
			}
			sv.push(y);
		}
		c = s[k];
	}
	cout << sv.top() << endl;
	return 0;
}