Your code runs in O(@a * @b) time because of the grep nested inside the foreach. The hash approach will run in O(A + @b), where A is the time to build the hash %seen out of the values of @a (probably about @a*log(@a)). Hashes should be faster if the data sets are somewhat large.

If the two lists of DNs are sorted initially, you can use a merge algorithm (as in the unix comm program). That is an optimal solution, because it takes O(@a + @b) time, and there's no way to solve the problem without examining everything in both lists. If you have to sort the lists first, that's more expensive than the hash method.

Update: Now that I'm getting analysis of algorithms paged back in, I'm thinking A should have been @a in the first paragraph. The hashtable has to expand about log(@a) times, but each time it only has to reconsider the elements that were previously inserted, so it's @a + @a/2 + @a/4 + ... = @a*2, which is proportional to @a.

In reply to Re: Re: using hash uniqueness to find the odd man by no_slogan
in thread using hash uniqueness to find the odd man by novitiate

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.