Initial data commit

Task/Permutations-Derangements/Java/permutations-derangements.java

@@ -0,0 +1,95 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class Derangement {
+
+    public static void main(String[] args) {
+        System.out.println("derangements for n = 4\n");
+        for (Object d  : (ArrayList)(derangements(4, false)[0])) {
+            System.out.println(Arrays.toString((int[])d));
+        }
+
+        System.out.println("\ntable of n vs counted vs calculated derangements\n");
+        for (int i = 0; i < 10; i++) {
+            int d = ((Integer)derangements(i, true)[1]).intValue();
+            System.out.printf("%d  %-7d %-7d\n", i, d, subfact(i));
+        }
+
+        System.out.printf ("\n!20 = %20d\n", subfact(20L));
+    }
+
+    static Object[] derangements(int n, boolean countOnly) {
+        int[] seq = iota(n);
+        int[] ori = Arrays.copyOf(seq, n);
+        long tot = fact(n);
+
+        List<int[]> all = new ArrayList<int[]>();
+        int cnt = n == 0 ? 1 : 0;
+
+        while (--tot > 0) {
+            int j = n - 2;
+            while (seq[j] > seq[j + 1]) {
+                j--;
+            }
+            int k = n - 1;
+            while (seq[j] > seq[k]) {
+                k--;
+            }
+            swap(seq, k, j);
+
+            int r = n - 1;
+            int s = j + 1;
+            while (r > s) {
+                swap(seq, s, r);
+                r--;
+                s++;
+            }
+
+            j = 0;
+            while (j < n && seq[j] != ori[j]) {
+                j++;
+            }
+            if (j == n) {
+                if (countOnly) {
+                    cnt++;
+                } else {
+                    all.add(Arrays.copyOf(seq, n));
+                }
+            }
+        }
+        return new Object[]{all, cnt};
+    }
+
+    static long fact(long n) {
+        long result = 1;
+        for (long i = 2; i <= n; i++) {
+            result *= i;
+        }
+        return result;
+    }
+
+    static long subfact(long n) {
+        if (0 <= n && n <= 2) {
+            return n != 1 ? 1 : 0;
+        }
+        return (n - 1) * (subfact(n - 1) + subfact(n - 2));
+    }
+
+    static void swap(int[] arr, int lhs, int rhs) {
+        int tmp = arr[lhs];
+        arr[lhs] = arr[rhs];
+        arr[rhs] = tmp;
+    }
+
+    static int[] iota(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("iota cannot accept < 0");
+        }
+        int[] r = new int[n];
+        for (int i = 0; i < n; i++) {
+            r[i] = i;
+        }
+        return r;
+    }
+}