无奈，看似没有任何技术含量，就是for嵌套循环，想得出结果太难

#include <stdio.h>

char reversion(char signal) {
    if (signal == '*') {
        return '/';
    } else if (signal == '/') {
        return '*';
    } else if (signal == '+') {
        return '-';
    } else if (signal == '-') {
        return '+';
    }
}

void get_equation(int A, int B, int C, int D, char x, char y, char z, char w, char* equ) {
    x = reversion(x);
    y = reversion(y);
    z = reversion(z);
    w = reversion(w);
    sprintf(equ,"a=%d%c(%d%c(%d%c(%d%ca)))",D,w,B,z,C,y,A,x);
}

float solve(int step) {
    float temp;
    for (float x = -9; x < 9*step; x++) {
        float ttemp;
        ttemp = (8*step-(5*step+(9*x/21)) - x);
        if (ttemp == 0) {
            return x/step;
        }
        if (x > 1 && (ttemp * temp < 0)) {
            break;
        }
        temp = ttemp;
    }
    solve(step*10);
}

float cal(float x, char opr, float y) {
    if (opr == '*') {
        return x*y;
    } else if (opr == '/') {
        return x/y;
    } else if (opr == '+') {
        return x+y;
    } else if (opr == '-') {
        return x-y;
    }
}


int main() {
    char equ[100] = {0};
    char x = '*';
    char y = '*';
    char z = '+';
    char w = '-';
    int A = 21;
    int B = 5;
    int C = 9;
    int D = 8;
    float a, b, c, d;
    get_equation(A, B, C, D, x, y, z, w, equ);
    printf("%s\n",equ);
    a = solve(1);
    b = cal(A, reversion(x), a);
    d = cal(C, reversion(y), b);
    c = cal(B, reversion(z), d);
    printf("a=%f\n", a);
    printf("b=%f\n", b);
    printf("c=%f\n", c);
    printf("d=%f\n", d);
    return 0;
}