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