Friday, December 10, 2010

find all anagrams from a large file of long words, in a single pass

Normalize each word using counting sort on the characters of the word. There's no competing alternative in this isolated part of the solution. Main part of the solution is sorting all words based on the normalized version. There are many alternative solutions to this sort.

desirable -- memory efficiency. create no or few objects besides the strings read in from the file.
desirable -- avoid the need to create/allocate linkNode or wrapper objects around original words. In c++, such a wrapper is smaller than in java, probably a pointer to the original word + a link pointer

Solution 1 -- insert into a sorted set, using a customized comparison (subclass binary_function<...>) based on the normalized string and something unique, such as line number or object address. After all the inserts, when you iterate the set, an anagram group would show up in a row. We can also keep a separate array or iterators (pointers) that point to the first word in each group -- emulating a multimap.

Without the something unique, the set would drop words.

I think insertion is inefficient since a normalized string is regenerated for the same word over and over. To overcome this, we could create a wrapper object with 2 fields -- normalized string + a pointer to the original word. However, if "SAT" is a very common normalized string then we create multiple copies of it.

Solution 2 -- separate chaining. Maintain a hashed map (faster) or sorted map keyed by the normalized strings, just a single copy for each normalized string. Value of each map entry is a linked list of the original words.

Solution -- multimap

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