freq [ i ] [ j ] : stores the frequency of a character ( = j ) in a String arr [ i ]
let freq = new Array ( N ) ; for ( let i = 0 ; i < N ; i ++ ) { freq [ i ] = new Array ( 256 ) ; for ( let j = 0 ; j < 256 ; j ++ ) { freq [ i ] [ j ] = 0 ; } }