form the vowel String ' vstr ' and consonant String ' cstr ' which contains characters in lexicographical order
for ( let i = 0 ; i < SIZE ; i ++ ) { let ch = String . fromCharCode ( i + 97 ) ; for ( let j = 1 ; j <= char_freq [ i ] ; j ++ ) { if ( isVowel ( ch ) ) { vstr += ch ; } else { cstr += ch ; } } }