Data commit

Task/Prime-triangle/Java/prime-triangle.java

@@ -0,0 +1,75 @@
+public class PrimeTriangle {
+    public static void main(String[] args) {
+        long start = System.currentTimeMillis();
+        for (int i = 2; i <= 20; ++i) {
+            int[] a = new int[i];
+            for (int j = 0; j < i; ++j)
+                a[j] = j + 1;
+            if (findRow(a, 0, i))
+                printRow(a);
+        }
+        System.out.println();
+        StringBuilder s = new StringBuilder();
+        for (int i = 2; i <= 20; ++i) {
+            int[] a = new int[i];
+            for (int j = 0; j < i; ++j)
+                a[j] = j + 1;
+            if (i > 2)
+                s.append(" ");
+            s.append(countRows(a, 0, i));
+        }
+        System.out.println(s);
+        long finish = System.currentTimeMillis();
+        System.out.printf("\nElapsed time: %d milliseconds\n", finish - start);
+    }
+
+    private static void printRow(int[] a) {
+        for (int i = 0; i < a.length; ++i) {
+            if (i != 0)
+                System.out.print(" ");
+            System.out.printf("%2d", a[i]);
+        }
+        System.out.println();
+    }
+
+    private static boolean findRow(int[] a, int start, int length) {
+        if (length == 2)
+            return isPrime(a[start] + a[start + 1]);
+        for (int i = 1; i + 1 < length; i += 2) {
+            if (isPrime(a[start] + a[start + i])) {
+                swap(a, start + i, start + 1);
+                if (findRow(a, start + 1, length - 1))
+                    return true;
+                swap(a, start + i, start + 1);
+            }
+        }
+        return false;
+    }
+
+    private static int countRows(int[] a, int start, int length) {
+        int count = 0;
+        if (length == 2) {
+            if (isPrime(a[start] + a[start + 1]))
+                ++count;
+        } else {
+            for (int i = 1; i + 1 < length; i += 2) {
+                if (isPrime(a[start] + a[start + i])) {
+                    swap(a, start + i, start + 1);
+                    count += countRows(a, start + 1, length - 1);
+                    swap(a, start + i, start + 1);
+                }
+            }
+        }
+        return count;
+    }
+
+    private static void swap(int[] a, int i, int j) {
+        int tmp = a[i];
+        a[i] = a[j];
+        a[j] = tmp;
+    }
+
+    private static boolean isPrime(int n) {
+        return ((1L << n) & 0x28208a20a08a28acL) != 0;
+    }
+}