52 - Andrew Stankevich's Contest, Warmup - 1001
Given a string consisting of brackets of two types find its longest substring that is a regular brackets sequence.
There are mutiple cases in the input file.
Each case contains a string containing only characters ( , ) , [ and ] . The length of the string does not exceed 100,000.
There is an empty line after each case.
Output the longest substring of the given string that is a regular brackets sequence.There should be am empty line after each case.