if 2 erase s [ i ] and replace next with 7
if ( s [ i ] <= '2' ) { s . erase ( i , 1 ) ; s [ i ] = '7' ; } else if ( s [ i ] == '3' ) s [ i ] = '2' ; else if ( s [ i ] <= '5' ) s [ i ] = '3' ; else if ( s [ i ] <= '7' ) s [ i ] = '5' ; else s [ i ] = '7' ; return ; } string primeDigits ( string s ) { for ( int i = 0 ; i < s . length ( ) ; i ++ ) {