1101
Both 1111 and 1001 is right.
But what's the "correct answer" ?
Who got Ac ? Tell me...

1111 is not correct
for input 1101

Edited by author 10.06.2008 16:09

I think  that it must be correct answer too.

In the problem there are words " one of the following modifications to a word may occur",I think it hope to means
that the following three modification one and only one must
occur.So ,1111 is a correct answer!because none of the three
modifications occur.

  The number is interesting.If the first number is 1,the last one must equal to 1;and the ...
  Be breaf,OK;
  char *a=new char[n];
  int the a[n],a[1] must equal to a[n-1]
               a[2] must equal to a[n-2]
                ...
   so if 1101,you can't decide wheather a[1] is changed(then answer is 1001) or a[2] is changed(then answer is 1111) .
  In this case 0001 also has two origner number 0000 or 1001
  .....

in my AC prog, the sequence is right to left

therefor in 1101 the order is: first digit is 1 and second is 0 ,...

1111 was not correct, because you can only change '1' to '0', can not change'0' to 1.(as the problem description said, maybe a '0' was replaced by '1').
So the Only Correct answer is 1001.