为什么我的代码提交结果每次会不一样

#include <stdio.h>  
#include <math.h>
 
int main()
{
    char x, a = 0, c = 0, b = 0, z, k[25500] = {0};
    for (int i = 0; k[i - 1] != '\n'; i++)
    {
        scanf("%c", &k[i]);
    }
    for (int i = 0; k[5*i] != '\n'; i++)
    {
        x = k[5 * i];
        z = k[5 * i + 3];  
        switch (z)
        {
        case 'a':
            z = a;
            break;
        case 'b':
            z = b;
            break;
        case 'c':
            z = c;
        default:
            break;
        }
        
        switch (x)
        {
        case 'a':
            a = z;
            break;
        case 'b':
            b = z;
            break;
        case 'c':
            c = z;
            break;
        }
    }
    printf("%c %c %c", a, b, c);
    return 0;
}