想想都不用想，肯定是用枚举。

#include<iostream>
using namespace std;
int main()
{
	int a, b, c, d, e, f;
	for (a=0; a < 2; a++)
	{
		for (b=0; b < 2; b++)
		{
			for (c=0; c < 2; c++)
			{
				for (d=0; d < 2; d++)
				{
					for (e=0; e < 2; e++)
					{
						for (f=0; f < 2; f++)
						{
							if ((a + b >= 1) && (a + d == 1) && (a + e + f == 2) && (c + d == 1) && (b + c != 1) && (d + e == 0 || d == 1))
							{
								if (a == 1)
								{
									cout << "a" << endl;
								}
								if (b == 1)
								{
									cout << "b" << endl;
								}
								if (c == 1)
								{
									cout << "c" << endl;
								}
								if (d == 1)
								{
									cout << "d" << endl;
								}
								if (e == 1)
								{
									cout << "e" << endl;
								}
								if (f == 1)
								{
									cout << "f" << endl;
								}
							}
						}
					}
				}
			}
		}
	}
	return 0;
}