problem with testing solution

it Is my code for( Ciel and A-B Problem ) and I did not understand why my answer is wrong…!

    #include <stdio.h>

int main()
{
 int a ;
 int b ;
int diff ;
scanf("%d", &a);
scanf("%d", &b);
diff = a - b ;
if  ((diff%100) == diff )
{
    if (diff <9 )
    {
        printf("%d\n",diff+1);
    }
    else
    {
      printf("%d\n",diff-1);
    }
}
else if ((diff%100) < 99 )
{
    printf("%d\n",diff+10);
}
else if ((diff%100) == 99 )
{

  printf("%d\n",diff-10);
}
    return 0;
}
best regards

Your code gives wrong output for this test case:

Input
298 100

Your Output
208

The actual difference is 198 and your code gives 208. As you can see, if differs at two places. Change your code to cover these type of cases.

However, this problem has a simpler solution.

Click to view

We want the result to differ at exactly one place. This means that we do not want any carry overs. Thus to avoid any carry overs, if the last digit of the answer is 9: subtract 1, else add 1. This way only the last digit will get changed.

if(diff % 10 == 9)
    diff -= 1;
else
    diff += 1;
//