text: Print characters having prime frequencies in order of occurrence | Java implementation of the approach ; Function to create Sieve to check primes ; false here indicates that it is not prime ; If prime [ p ] is not changed , then it is a prime ; Update all multiples of p , set them to non - prime ; Function to print the prime frequency characters in the order of their occurrence ; Function to create Sieve to check primes ; To store the frequency of each of the character of the string ; Initialize all elements of freq [ ] to 0 ; Update the frequency of each character ; Traverse str character by character ; If frequency of current character is prime ; Driver code
code: class GFG { static int SIZE = 26 ; static void SieveOfEratosthenes ( boolean [ ] prime , int p_size ) { prime [ 0 ] = false ; prime [ 1 ] = false ; for ( int p = 2 ; p * p <= p_size ; p ++ ) { if ( prime [ p ] ) { for ( int i = p * 2 ; i < p_size ; i += p ) prime [ i ] = false ; } } } static void printChar ( String str , int n ) { boolean [ ] prime = new boolean [ n + 1 ] ; for ( int i = 0 ; i < n + 1 ; i ++ ) prime [ i ] = true ; SieveOfEratosthenes ( prime , str . length ( ) + 1 ) ; int [ ] freq = new int [ SIZE ] ; for ( int i = 0 ; i < SIZE ; i ++ ) freq [ i ] = 0 ; for ( int i = 0 ; i < n ; i ++ ) freq [ str . charAt ( i ) - ' a ' ] ++ ; for ( int i = 0 ; i < n ; i ++ ) { if ( prime [ freq [ str . charAt ( i ) - ' a ' ] ] ) { System . out . print ( str . charAt ( i ) ) ; } } } public static void main ( String [ ] args ) { String str = " geeksforgeeks " ; int n = str . length ( ) ; printChar ( str , n ) ; } }