有未知数的表达式

#include <iostream>
#include <math.h>
#include <cstring>
using namespace std;

char buf[35];
int result;
int main()
{
	int calcu(int i, int j);
	memset(buf, 0, sizeof(buf));
	while(cin.getline(buf, 35))
	{
		result = 0;
		int i, j, result = 0;
		cout << calcu(0, strlen(buf)) << endl;
	}
	return 0;
}

int getNum(int begin, int end)
{
	int result = 0;
	for(int i = begin; i < end; i++)
	{
		result = result * 10 + (buf[i] - '0');
	}
	return result;
}

int calcu(int begin, int end)
{
	int i, j;
	for(i = begin; i < end; i++)
	{
		if(buf[i] == '+')
		{
			return (calcu(begin, i) + calcu(i + 1, end));
		}
	}
	for(i = end - 1; i >= begin; i--)
	{
		if(buf[i] == '-')
		{
			return (calcu(begin, i) - calcu(i + 1, end));
		}
	}
	int xmark = 0;
	int r = 0;
	for(i = begin; i < end; i++)
	{
		if(buf[i] == 'x')
		{
			xmark = 1;
			if(buf[i + 1] == '^')
			{
				r = pow((double)3, getNum(i + 2, end));
				if(i != begin)
				{
					r *= getNum(begin, i);
				}
			}
			else if(begin + 1 == end)
			{
				r = 3;
			}
			else
			{
				r = 3 * getNum(begin, i);
			}
		}
	}
	if(!xmark)
	{
		r = getNum(begin, end);
	}
	return r;
}