java - All combinations of alphanumeric string, better way? -

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the input "alphanumeric" function string consists of alphanumeric characters lower case, example "hello123hello". want able check upper/lower case letter combinations string through check( ) function. (eg. hello123hello 1 of combinations checked). have written code in java store matching string arraylist, know if there better way without arraylist. also, correct in saying worst case runtime of o(2^n)? note: check function returns either true or false, depending on whether correct string passed function.

public static string alphanumeric(string input) {     arraylist<string> list = new arraylist<string>();     alphahelper(input, "", list);     return list.get(0); }  private static void alphahelper(string in, string current, arraylist<string> list) {     if (in.length() == 0) {         if (check(current)) {             list.add(current);         }     } else if (character.isletter(in.charat(0))) {         alphahelper(in.substring(1),current+in.substring(0,1).tolowercase(),list);         alphahelper(in.substring(1),current+in.substring(0,1).touppercase(),list);     } else if (character.isdigit(in.charat(0))) {         alphahelper(in.substring(1),current+in.substring(0,1),list);     } else {         return;     } }

if want remove arraylist without changing basic algorithm, can this:

public static string alphanumeric(string input) {     return alphahelper(input, ""); }  private static string alphahelper(string in, string current) {     string result = null;      if (check(current)) {         result = current;     } else if (character.isletter(in.charat(0))) {         result = alphahelper(in.substring(1),current+in.substring(0,1).tolowercase());         if (result == null) result = alphahelper(in.substring(1),current+in.substring(0,1).touppercase());     } else if (character.isdigit(in.charat(0))) {         result = alphahelper(in.substring(1),current+in.substring(0,1));     }      return result; }

yes o(2^n), , can't see offhand how improve on if can't original string directly.

if don't need check substrings (i.e. care case variations of entire string) improve algorithm not testing substrings, still o(2^n).


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