Damn - everyone beat me to it, but I have a nice working example :)

http://www.fiveminuteargument.com/so-727707

The key is identifying the rules, which are actually quite simple:

• Build the string char-by-char
• At a given point in the string
  • if brackets in string so far balance (includes empty str), add an open bracket and recurse
  • if all open brackets have been used, add a close bracket and recurse
  • otherwise, recurse twice, once for each type of bracket
• Stop when you get to the end :-)