To store the
HashMap < Character , Integer > occ = new HashMap < Character , Integer > ( ) ; for ( int i = 0 ; i < len ; i ++ ) if ( occ . containsKey ( arr [ i ] ) ) { occ . put ( arr [ i ] , occ . get ( arr [ i ] ) + 1 ) ; } else { occ . put ( arr [ i ] , 1 ) ; }