Occur [ i ] stores the indices of char ( ' a ' + i ) in string s
var occur = Array . from ( Array ( 26 ) , ( ) => new Array ( ) ) ; for ( var x = 0 ; x < n ; x ++ ) { var ch = s [ x ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ; occur [ ch ] . push ( x ) ; }