Create a map to store the frequency of each character
HashMap < Character , Integer > m = new HashMap < Character , Integer > ( ) ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { char p = s . charAt ( i ) ; Integer count = m . get ( p ) ; if ( count == null ) { count = 0 ; m . put ( p , 1 ) ; } else m . put ( p , count + 1 ) ; }