Store the frequency of each element from the array
for ( int i = 0 ; i < n ; i ++ ) { if ( freq . ContainsKey ( arr [ i ] ) ) { freq [ arr [ i ] ] = freq [ arr [ i ] ] + 1 ; } else { freq . Add ( arr [ i ] , 1 ) ; } } HashSet < int > uniqueFreq = new HashSet < int > ( ) ;