Common APT

Problem Statement

Two strings have a letter in common if it appears in both strings. The position of letter is not important for it to be counted as common. Once a particular letter is counted as in common, it cannot be counted again. For example, an o appearing twice in one word but only once in the other counts only as one letter in common. However, an o appearing twice both words counts as two letters in common.

Write a function that takes two strings and returns the number of letters they have in common.

Hint: you will need some way to avoid counting a letter more than once if it is in common to both words. To do this, replace each letter with a character that does not normally occur in words (e.g, "@") once you have examined it.