Question

This seems to be extremely simple, but if I’ve got a mental block.
I have a string which contains, say, only ‘_’ and ‘x’, and I need to find backwards positions of all x-sequences:

xxx___xxx___xxx
___x__xxx_xxx__

What is the fastest approach? Should I use KMP or BM or it’s an overkill?

Answer 1

You can scan the string letter by letter. Here’s the pseudocode in python:

 prev = ''
 # enumerate(collection) enumerates collection elements along with their indices
 # in the form of tuple (index, element)
 # in python strings are collections of characters
 for i, c in enumerate(string):  
     if c == 'x' and c != prev:
          print "found x sequence at position %d" % i # (this prints out the index)
     prev = c