dp [ i ] [ j ] stores the count of subsequence with ' a ' + i and ' a ' + j character
var dp = Array . from ( Array ( 26 ) , ( ) => Array ( 26 ) . fill ( 0 ) ) ; for ( var i = 0 ; i < s . length ; ++ i ) { for ( var j = 0 ; j < 26 ; j ++ ) {