Store counts of all elements .
HashMap < Integer , Integer > count = new HashMap < Integer , Integer > ( ) ; for ( int i = 0 ; i < n ; i ++ ) { if ( count . containsKey ( a [ i ] ) ) { count . put ( a [ i ] , count . get ( a [ i ] ) + 1 ) ; } else { count . put ( a [ i ] , 1 ) ; } } }