void str_cat(char *s1, char *s2)
{
    int i = 0, j = 0;
    while (s1[i++]);
    i--;
    while (s1[i++] = s2[j++]);
}

void str_cat (char *s1, char *s2){
int i = 0, j = 0;
    while (s1[i])
        i++;
    while (s2[j])
        s1[i++] = s2[j++];
}