Store all characters and their frequencies in dictionary
Map < Character , Integer > d = new HashMap < Character , Integer > ( ) ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { if ( d . containsKey ( s . charAt ( i ) ) ) { d . put ( s . charAt ( i ) , d . get ( s . charAt ( i ) ) + 1 ) ; } else { d . put ( s . charAt ( i ) , 1 ) ; } }