经典算法：穷举法解决24点问题

#define NUM_SIZE 4
#define OP_SIZE 4

const double neglible = 10e-7;
void twentyFour(int x, int y, int z, int a);
bool tryTwentyFour(int* n, double(*op[])(double, double));

//基本运算
double add(double a, double b);
double sub(double a, double b);
double mul(double a, double b);
double divs(double a, double b);

void twentyFour(int x, int y, int z, int a)
{
    int number[] = { x, y, z, a };
    double(*op[4])(double a, double b) = { add, sub, mul, divs };
    bool judge = false;

    sort(number, number + 4);

    while (next_permutation(number, number + 4))  //取所有的可能排列
    {
        if (tryTwentyFour(number, op))
            judge = true;
    }

    if (!judge)
&n