I tried hard and hard but was getting runtime error: access violation again and again because of not taking proper size of array. at last I used a[128*1024] by a solution of a discussion. and it worked. but how it was done? I mean I didn't get the clue line of question " A size of the input stream does not exceed 256 KB. "

can you help me?

256KB of chars - like "1.2 11 1". Here are 3 numbers take 8 bytes.
Size of minimal double representation is 2 bytes - "1 ". So 256K chars can be 128K doubles.