Task: Bell numbers
C#: using System;
using System.Numerics;

namespace BellNumbers {
    public static class Utility {
        public static void Init<T>(this T[] array, T value) {
            if (null == array) return;
            for (int i = 0; i < array.Length; ++i) {
                array[i] = value;
            }
        }
    }

    class Program {
        static BigInteger[][] BellTriangle(int n) {
            BigInteger[][] tri = new BigInteger[n][];
            for (int i = 0; i < n; ++i) {
                tri[i] = new BigInteger[i];
                tri[i].Init(BigInteger.Zero);
            }
            tri[1][0] = 1;
            for (int i = 2; i < n; ++i) {
                tri[i][0] = tri[i - 1][i - 2];
                for (int j = 1; j < i; ++j) {
                    tri[i][j] = tri[i][j - 1] + tri[i - 1][j - 1];
                }
            }
            return tri;
        }

        static void Main(string[] args) {
            var bt = BellTriangle(51);
            Console.WriteLine("First fifteen and fiftieth Bell numbers:");
            for (int i = 1; i < 16; ++i) {
                Console.WriteLine("{0,2}: {1}", i, bt[i][0]);
            }
            Console.WriteLine("50: {0}", bt[50][0]);
            Console.WriteLine();
            Console.WriteLine("The first ten rows of Bell's triangle:");
            for (int i = 1; i < 11; ++i) {
                
                var it = bt[i].GetEnumerator();
                Console.Write("[");
                if (it.MoveNext()) {
                    Console.Write(it.Current);
                }
                while (it.MoveNext()) {
                    Console.Write(", ");
                    Console.Write(it.Current);
                }
                Console.WriteLine("]");
            }
        }
    }
}

Java: 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();
        }
    }
}