17-recursive_backtracking

# The arrangements may be output in any order example

• Notes
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words that can be formed from unique letters in that string. The arrangements may be output in any order. – Example: combinations("GOOGLE", 3) outputs the sequence of lines at right. To simplify the problem, you may assume that the string s contains at least k unique characters. LEG LEO LGE LGO LOE LOG OEG OEL OGE OGL OLE OLG EGL EGO ELG ELO EOG EOL GEL GEO GLE GLO GOE GOL

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12 Initial attempt public static void combinations(String s, int length) { combinations(s, "", length); } private static void combinations(String s, String chosen, int length) { if (length == 0) { System.out.println(chosen) ; // base case: no choices left } else { for (int i = 0; i < s.length(); i++) { String ch = s.substring(i, i + 1); if (!chosen.contains(ch)) { String rest = s.substring(0, i) + s.substring(i + 1); combinations(rest, chosen + ch, length - 1); } } } } Problem: Prints same string multiple times.
13 Exercise solution public static void combinations(String s, int length) { Set<String> all = new TreeSet<String>(); combinations(s, "", all , length); for (String comb : all) { System.out.println(comb); } } private static void combinations(String s, String chosen, Set<String> all , int length) { if (length == 0) { all.add(chosen) ; // base case: no choices left } else { for (int i = 0; i < s.length(); i++) { String ch = s.substring(i, i + 1); if (!chosen.contains(ch)) { String rest = s.substring(0, i) + s.substring(i + 1); combinations(rest, chosen + ch, all, length - 1); } } } }
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• Spring '08
• SR
• Computer Science, Backtracking, private static void, MARTY MARYT, String ch, static void combinations

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