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