start of solution for #24, far too slow though

src/Problem024.java

@@ -0,0 +1,85 @@
+import java.util.ArrayList;
+import java.util.Collections;
+
+/**
+ * File: Problem024.java
+ * Date created: Jul 26, 2013
+ * 
+ * @author agargiulo
+ */
+
+/**
+ * @version 0.5
+ * @author agargiulo
+ *         A permutation is an ordered arrangement of objects. For example, 3124
+ *         is one possible permutation of the digits 1, 2, 3 and 4. If all of
+ *         the permutations are listed numerically or alphabetically, we call it
+ *         lexicographic order. The lexicographic permutations of 0, 1 and 2
+ *         are:
+ * 
+ *         012 021 102 120 201 210
+ * 
+ *         What is the millionth lexicographic permutation of the digits 0, 1,
+ *         2, 3, 4, 5, 6, 7, 8 and 9?
+ * 
+ */
+public class Problem024
+{
+
+	private static ArrayList<Integer>	permutations;
+
+	private static int arrToInt(int[] digits)
+	{
+		/*
+		 * [1,2,3,4,5] ==> 12345
+		 * 1 * 10^(arr.len -1)
+		 */
+		int powOfTen, ret = 0;
+		for (int i = 0; i < digits.length; i++)
+		{
+			powOfTen = digits.length - (i + 1);
+			ret += digits[i] * (int) Math.pow(10, powOfTen);
+		}
+		return ret;
+
+	}
+
+	private static void getPermutations(int[] digits, int index)
+	{
+		if (index == digits.length)
+		{
+			if (!permutations.contains(arrToInt(digits)))
+			{
+				permutations.add(arrToInt(digits));
+			}
+			return;
+		}
+		for (int i = index; i < digits.length; i++)
+		{
+			int[] permutation = digits.clone();
+			permutation[index] = digits[i];
+			permutation[i] = digits[index];
+			getPermutations(permutation, index + 1);
+		}
+	}
+
+	/**
+	 * @param args
+	 */
+	public static void main(String[] args)
+	{
+		permutations = new ArrayList<Integer>(3628800);
+		int[] digits =
+		{0,1,2,3,4,5,6,7,8,9};
+		getPermutations(digits, 0);
+		Collections.sort(permutations);
+		System.out.println("number of permutations: " + permutations.size());
+		System.out.println(permutations);
+		/*
+		 * System.out
+		 * .print("The millionth lexicographic permutation of 0123456789 is ");
+		 * System.out.println(permutations.get(999999));
+		 */
+	}
+
+}