Kata

This kata concerns a string composed of 'chunks'.

A chunk is either:

• A single alphabetic character

or:

• Two chunks enclosed in square brackets [ and ]

A 'chunky string' is a string consisting of exactly one chunk. Here are some examples of chunky strings:

'x'
'[xy]'
'[x[yy]]'
'[[xy]y]'
'[[ab][ab]]'
'[c[o[d[e[w[a[rs]]]]]]]'

For this task you will be given a valid chunky string from which all the closing brackets have been removed. You must restore the brackets and return the original valid chunky string.

For example:

Input               Restored String
'x'                 'x'
'[xy'               '[xy]'
'[x[yy'             '[x[yy]]'
'[[xyy'             '[[xy]y]'
'[[ab[ab'           '[[ab][ab]]'
'[c[o[d[e[w[a[rs'   '[c[o[d[e[w[a[rs]]]]]]]'

In the tests all input strings will be valid chunky strings (with closing brackets removed).