How to Change Rows of Arrays into Opposite

I have a Sheet with Column A being City and Column B being District, like so:

I would like to transform this into the "opposite":

I’m pretty sure it’s simple. Thanks !

>Solution :

use:

=ARRAYFORMULA(QUERY(SPLIT(REGEXREPLACE(TRIM(FLATTEN(
 QUERY(QUERY(SPLIT(FLATTEN(SPLIT(B2:B, ",")&"¤×"&A2:A&","), "×"), 
 "select max(Col2) group by Col2 pivot Col1"),,9^9))), 
 ",$", ), "¤ ", 0), "where Col2 is not null", 0))