Tetris-ing an array

Question

Consider the following array:

/www/htdocs/1/sites/lib/abcdedd
/www/htdocs/1/sites/conf/xyz
/www/htdocs/1/sites/conf/abc/         


        

Answer 1

Write a function longest_common_prefix that takes two strings as input. Then apply it to the strings in any order to reduce them to their common prefix. Since it is associative and commutative the order doesn't matter for the result.

This is the same as for other binary operations like for example addition or greatest common divisor.