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
harsh_verma: 2016-10-14 18:55:28

if u are getting SIGXFSZ see if u have written while(scanf("%d",&n)) change it to while(scanf("%d",&n)==1) or while(scanf("%d",&n)!=EOF)

lalywr: 2016-10-01 22:26:45

Finallllly AC :D ! First KMP !
For EOF , use while(scanf"%d",&n)!=EOF)
Probably the worst ever explained question :( Costed a few WA's

pd17: 2016-09-19 17:32:42

My code is giving SIGXFSZ runtime error. How to remove it

davidgalehouse: 2016-09-17 05:02:35

KMP was faster than naive for me in C#, but not by a huge amount. Regardless, if you're going for performance you'll want to use StringBuilder. Oh yeah, didn't try to 'stream' the input. Just used ReadLine.

Last edit: 2016-09-17 05:04:58
Elton Rawn: 2016-09-12 06:36:47

For this testcase, my code doesn't pass, yet it got ac.
I am quite surprised, as this kind of cases should've been included.

sri: 2016-09-01 20:20:06

length of pattern <=1000010
length of text <=1000010
First KMP!

Last edit: 2016-09-01 20:20:20
shriyam_011: 2016-07-17 13:52:10

thanks @amit Doshi for your explanation of \n format..
in cases where you found a match print an empty line after the ans for the case..
if no match is found print nothing...

citransvostok: 2016-07-16 19:25:27

my kmp solution gave 0.54s and pure stl gave 0.52, is there something wrong with my kmp?

And testcases are very loose, i think, i had a major bug, but it passed

xinnix: 2016-07-10 15:05:30

No need for table...
And please when giving hints in comments, be careful as while trying to not tell too much, you don't even tell what you are giving hint about. Here, test cases and haystack are both unknowns.....

avisheksanvas: 2016-07-03 07:59:58

Stl :)
Remember -> std::string::npos
But try to do it with KMP, you'll feel good!

Last edit: 2016-07-04 08:16:31

Added by:mima
Time limit:5s
Source limit:50000B
Memory limit:1536MB
Cluster: Cube (Intel G860)
Resource:the problem was phrased and test data was supplied by Mr. Maciej 'hawk' Jarzębski