用函数实现strcat功能

#include<stdio.h>

char *cat(char *d, char *s)
{
char *q = d;
char *b = s;
int i = 0; 
int j = 0;
if (q == NULL || b == NULL)
{
return NULL;
}
    else 
{
while (*(q + i) != '\0')
{
i++;
}


while (*(b + j) != '\0')
{
*(q + i) = *(b + j);
        i++;
j++;
}
   *(q + i)  = '\0';
printf("%s\n", q);
return q;
}
}
main()
{
    char a[20] = "hello";
printf("%s\n", a);
char b[] = "world";
char *c = cat(a, b);
printf("%s\n", a);
printf("%s\n", c);
printf("awsdasdsa");
return 0;
}