Created
September 7, 2013 09:40
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Derangement, permutation of integer array where for any i, a[i] != i
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import java.util.*; | |
public class Derangement { | |
public static void main(String[] args) { | |
List<Integer> list = new ArrayList<>(); | |
for (int i=0; i<4; ++i) { | |
list.add(i); | |
} | |
for (List<Integer> li : derangement(list)) { | |
System.out.println(li); | |
} | |
} | |
public static <T> List<List<T>> derangement(List<T> elements) { | |
List<List<T>> list = new ArrayList<>(); | |
if (elements.size() == 0) { | |
list.add(new ArrayList<T>()); | |
} else { | |
backtrack(elements, new int[elements.size()], 0, new BitSet(elements.size()), list); | |
} | |
return list; | |
} | |
private static <T> void backtrack(final List<T> elements, | |
final int[] buffer, | |
int k, | |
BitSet bs, | |
List<List<T>> result) { | |
if (k == elements.size()) { | |
List<T> range = new ArrayList<>(); | |
for (int i : buffer) { | |
range.add(elements.get(i)); | |
} | |
result.add(range); | |
} else { | |
for (int i=0; i<elements.size(); ++i) { | |
if (!bs.get(i) && i != k) { | |
bs.set(i); | |
buffer[k] = i; | |
backtrack(elements, buffer, k+1, bs, result); | |
bs.set(i, false); | |
} | |
} | |
} | |
} | |
} |
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