Problem
You know a password is well-ordered string. Well-ordered string means that the order of the characters is in an alphabetical increasing order. Like “abEm” is a well-ordered number. However, “abmE” is not a well-order number. Given an input# that tells you also how many digits are in the password, print all possible
Solution
Recursive + DFS
1 public static List<List<String>> hackCode(int num) { 2 List<List<String>> res = new ArrayList<List<String>>(); 3 if (num < 0) { 4 return res; 5 } 6 7 List<String> ls = new ArrayList<String>(); 8 helper(res, ls, 0, num, 0); 9 return res; 10 } 11 12 public static void helper(List<List<String>> res, List<String> ls, int prev, 13 int num, int pos) { 14 if (pos == num) { 15 res.add(new ArrayList<String>(ls)); 16 return; 17 } 18 19 for (int i = prev; i < 26; i++) { 20 char c = (char) (‘a‘ + i); 21 ls.add(String.valueOf(c)); 22 helper(res, ls, i + 1, num, pos + 1); 23 ls.remove(ls.size() - 1); 24 } 25 }
原文：http://www.cnblogs.com/superbo/p/4111922.html