Data commit

Task/Bell-numbers/Java/bell-numbers.java

@@ -0,0 +1,61 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class Bell {
+    private static class BellTriangle {
+        private List<Integer> arr;
+
+        BellTriangle(int n) {
+            int length = n * (n + 1) / 2;
+            arr = new ArrayList<>(length);
+            for (int i = 0; i < length; ++i) {
+                arr.add(0);
+            }
+
+            set(1, 0, 1);
+            for (int i = 2; i <= n; ++i) {
+                set(i, 0, get(i - 1, i - 2));
+                for (int j = 1; j < i; ++j) {
+                    int value = get(i, j - 1) + get(i - 1, j - 1);
+                    set(i, j, value);
+                }
+            }
+        }
+
+        private int index(int row, int col) {
+            if (row > 0 && col >= 0 && col < row) {
+                return row * (row - 1) / 2 + col;
+            } else {
+                throw new IllegalArgumentException();
+            }
+        }
+
+        public int get(int row, int col) {
+            int i = index(row, col);
+            return arr.get(i);
+        }
+
+        public void set(int row, int col, int value) {
+            int i = index(row, col);
+            arr.set(i, value);
+        }
+    }
+
+    public static void main(String[] args) {
+        final int rows = 15;
+        BellTriangle bt = new BellTriangle(rows);
+
+        System.out.println("First fifteen Bell numbers:");
+        for (int i = 0; i < rows; ++i) {
+            System.out.printf("%2d: %d\n", i + 1, bt.get(i + 1, 0));
+        }
+
+        for (int i = 1; i <= 10; ++i) {
+            System.out.print(bt.get(i, 0));
+            for (int j = 1; j < i; ++j) {
+                System.out.printf(", %d", bt.get(i, j));
+            }
+            System.out.println();
+        }
+    }
+}