pushing left adjacent elelemt for index ' k '
if ( k - 1 >= 0 && ! edge [ s [ k ] . charCodeAt ( 0 ) - 97 ] [ s [ k - 1 ] . charCodeAt ( 0 ) - 97 ] ) gr [ s [ k ] . charCodeAt ( 0 ) - 97 ] . push ( s [ k - 1 ] . charCodeAt ( 0 ) - 97 ) , edge [ s [ k ] . charCodeAt ( 0 ) - 97 ] [ s [ k - 1 ] . charCodeAt ( 0 ) - 97 ] = 1 ;