To store frequency of 4 characters
HashMap < Character , Integer > mp = new HashMap < > ( ) ; if ( mp . containsKey ( A . charAt ( i ) ) ) mp . put ( A . charAt ( i ) , mp . get ( A . charAt ( i ) ) + 1 ) ; else mp . put ( A . charAt ( i ) , 1 ) ; if ( mp . containsKey ( A . charAt ( n - i - 1 ) ) ) mp . put ( A . charAt ( n - i - 1 ) , mp . get ( A . charAt ( n - i - 1 ) ) + 1 ) ; else mp . put ( A . charAt ( n - i - 1 ) , 1 ) ; if ( mp . containsKey ( B . charAt ( i ) ) ) mp . put ( B . charAt ( i ) , mp . get ( B . charAt ( i ) ) + 1 ) ; else mp . put ( B . charAt ( i ) , 1 ) ; if ( mp . containsKey ( B . charAt ( n - i - 1 ) ) ) mp . put ( B . charAt ( n - i - 1 ) , mp . get ( B . charAt ( n - i - 1 ) ) + 1 ) ; else mp . put ( B . charAt ( n - i - 1 ) , 1 ) ; int sz = mp . size ( ) ;