Map data structure to count the frequency of each element
HashMap < Integer , Integer > freq = new HashMap < Integer , Integer > ( ) ; for ( int i = 0 ; i < n ; i ++ ) { if ( freq . containsKey ( arr [ i ] ) ) { freq . put ( arr [ i ] , freq . get ( arr [ i ] ) + 1 ) ; } else { freq . put ( arr [ i ] , 1 ) ; } } int len = 0 ; for ( int i = 1 ; i <= n ; i ++ ) {