NHAY - A Needle in the Haystack

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Write a program that finds all occurences of a given pattern in a given input string. This is often referred to as finding a needle in a haystack.

The program has to detect all occurences of the needle in the haystack. It should take the needle and the haystack as input, and output the positions of each occurence, as shown below. The suggested implementation is the KMP algorithm, but this is not a requirement. However, a naive approach will probably exceed the time limit, whereas other algorithms are more complicated... The choice is yours.


The input consists of a number of test cases. Each test case is composed of three lines, containing:

  • the length of the needle,
  • the needle itself,
  • the haystack.

The length of the needle is only limited by the memory available to your program, so do not make any assumptions - instead, read the length and allocate memory as needed. The haystack is not limited in size, which implies that your program should not read the whole haystack at once. The KMP algorithm is stream-based, i.e. it processes the haystack character by character, so this is not a problem.

The test cases come one after another, each occupying three lines, with no additional space or line breaks in between.


For each test case your program should output all positions of the needle's occurences within the haystack. If a match is found, the output should contain the position of the first character of the match. Characters in the haystack are numbered starting with zero.

For a given test case, the positions output should be sorted in ascending order, and each of these should be printed in a separate line. For two different test cases, the positions should be separated by an empty line.


Sample input:
Sample output:


Note the double empty line in the output, which means that no match was found for the second test case.

Warning: large Input/Output data, be careful with certain languages

hide comments
yead_025: 2017-11-28 23:53:01

try rolling hash!

Last edit: 2017-11-29 17:16:06
themast3r: 2017-11-04 17:40:05

AC in one go!
1. The output format on SPOj-Toolkit is wrong, so do not panic if their output does not matches yours.
2. There is nothing like "overlapping cases are not considered" and all that.
3. No need to use scanf and printf. You will do fine with cin and cout as long as you do:
ios_base :: sync_with_stdio(false);
4. Yes, you can pass all the test case with string search functions from C++ STL but that is not the point of this question.
5. For C++ to read EOF simply use if(cin >> x).

Last edit: 2017-11-04 17:49:20
themast3r: 2017-11-04 13:31:51

Last edit: 2017-11-04 17:40:19
algolover: 2017-10-18 22:53:39

Hi i dont know expression in c++ for terminating this input loop can anyone help?

sid_somani: 2017-08-28 20:38:49

I used 2d dp and got AC

Habibur Rahman Habib: 2017-08-16 23:04:40

Bla Bla Bla

nessaa_05: 2017-08-06 12:04:36

cute babe the question is :P

imshubhamk: 2017-07-28 06:45:42

AC in one go.................
used std::search() STL c++ algorithm..

anubhav1772: 2017-07-05 21:59:37

Worst explained problem.Costed me 2 WA !

aronzx: 2017-06-18 19:54:56

Learnt KMP. AC in one GO :)

Added by:mima
Time limit:5s
Source limit:50000B
Memory limit:1536MB
Cluster: Cube (Intel G860)