map that marks the occurrence of a number
var m = new Map ( ) ; for ( var i = 0 ; i < l ; i ++ ) { if ( m . has ( s . charCodeAt ( i ) - 48 ) ) m . set ( s . charCodeAt ( i ) - 48 , m . get ( s . charCodeAt ( i ) - 48 ) + 1 ) ; else m . set ( s . charCodeAt ( i ) - 48 , 1 ) ; }