Inversion count done!

Pretty challenging but a lot of fun and very satisfying. Decided to
implement MergeSort first without worrying about inversion count, then
went from there. Lesson learned: use the debugger! :) Also passing by
reference in Java can be convenient.

2 files changed, 159 insertions, 0 deletions

diff --git a/AlgoDesignAndTechniqueEdxJava/sources/InversionCount.java b/AlgoDesignAndTechniqueEdxJava/sources/InversionCount.java
new file mode 100644
index 0000000..dd678fc
--- /dev/null
+++ b/AlgoDesignAndTechniqueEdxJava/sources/InversionCount.java
@@ -0,0 +1,109 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Scanner;
+import java.util.stream.Collectors;
+
+public class InversionCount {
+
+	public static void main(String[] args) {
+		Scanner scanner = new Scanner(System.in);
+		int n = scanner.nextInt();
+		int[] a = new int[n];
+		for (int i = 0; i < n; i++) {
+			a[i] = scanner.nextInt();
+		}
+//		int[] b = new int[n];
+		System.out.println(getInversionCountFromArray(a));
+		scanner.close();
+	}
+
+	public static long getNumberOfInversions(int[] a, int[] b, int left, int right) {
+		long numberOfInversions = 0;
+		if (right <= left + 1) {
+			return numberOfInversions;
+		}
+		int middle = (left + right) / 2;
+		numberOfInversions += getNumberOfInversions(a, b, left, middle);
+		numberOfInversions += getNumberOfInversions(a, b, middle, right);
+		// write your code here
+		return numberOfInversions;
+	}
+
+	public static long getInversionCountFromArray(int[] a) {
+		List<Integer> aList = Arrays.stream(a).boxed().collect(Collectors.toList());
+		List<Integer> D = new ArrayList<Integer>();
+		long numberOfInversions = getInversionCountFromList(aList, D);
+		return numberOfInversions;
+	}
+
+	private static long getInversionCountFromList(List<Integer> A, List<Integer> D) {
+		long numberOfInversions = 0;
+		if (A.size() == 1) {
+			D.add(A.get(0));
+			return 0;
+		}
+		int m = A.size() / 2;
+		List<Integer> D1 = new ArrayList<Integer>();
+		numberOfInversions += getInversionCountFromList(new ArrayList<Integer>(A.subList(0, m)), D1);
+		List<Integer> D2 = new ArrayList<Integer>();
+		numberOfInversions += getInversionCountFromList(new ArrayList<Integer>(A.subList(m, A.size())), D2);
+//		long mergeCount = mergeInversionCount(new ArrayList<Integer>(A.subList(0, m)), new ArrayList<Integer>(A.subList(m, A.size())));
+		long mergeCount = mergeInversionCount(D1, D2, D);
+		return numberOfInversions + mergeCount;
+	}
+
+	private static long mergeInversionCount(List<Integer> B, List<Integer> C, List<Integer> D) {
+		long count = 0;
+		while (B.size() != 0 && C.size() != 0) {
+			int b = B.get(0);
+			int c = C.get(0);
+			if (b <= c) {
+				D.add(b);
+				B.remove(0);
+			} else {
+				count += B.size();
+				D.add(c);
+				C.remove(0);
+			}
+		}
+		D.addAll(B);
+		D.addAll(C);
+		return count;
+	}
+
+	public static int[] mergeSortArray(int[] a) {
+		List<Integer> aList = Arrays.stream(a).boxed().collect(Collectors.toList());
+		List<Integer> sortedList = mergeSortList(aList);
+		return sortedList.stream().mapToInt(i -> i).toArray();
+	}
+
+	private static List<Integer> mergeSortList(List<Integer> A) {
+		if (A.size() == 1)
+			return A;
+		int m = A.size() / 2;
+		List<Integer> B = mergeSortList(new ArrayList<Integer>(A.subList(0, m)));
+		List<Integer> C = mergeSortList(new ArrayList<Integer>(A.subList(m, A.size())));
+		List<Integer> aPrime = mergeList(B, C);
+		return aPrime;
+	}
+
+	private static List<Integer> mergeList(List<Integer> B, List<Integer> C) {
+		List<Integer> D = new ArrayList<>();
+		while (B.size() != 0 && C.size() != 0) {
+			int b = B.get(0);
+			int c = C.get(0);
+			if (b <= c) {
+				D.add(b);
+				B.remove(0);
+			} else {
+				D.add(c);
+				C.remove(0);
+			}
+		}
+		D.addAll(B);
+		D.addAll(C);
+		return D;
+	}
+
+}
diff --git a/AlgoDesignAndTechniqueEdxJava/tests/InversionCountTest.java b/AlgoDesignAndTechniqueEdxJava/tests/InversionCountTest.java
new file mode 100644
index 0000000..483cd23
--- /dev/null
+++ b/AlgoDesignAndTechniqueEdxJava/tests/InversionCountTest.java
@@ -0,0 +1,50 @@
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class InversionCountTest {
+
+	@Test
+	void testInversionCount0() {
+		int[] a = {2, 3, 9, 2, 9};
+		assertEquals(2, InversionCount.getInversionCountFromArray(a));
+	}
+
+	@Test
+	void testInversionCount1() {
+		int[] a = {1, 1, 1, 1, 1};
+		assertEquals(0, InversionCount.getInversionCountFromArray(a));
+	}
+
+	@Test
+	void testInversionCount2() {
+		int[] a = {6, 5, 4, 3, 2, 1};
+		assertEquals(15, InversionCount.getInversionCountFromArray(a));
+	}
+
+	@Test
+	void testInversionCount3() {
+		int[] a = {1, 1, 1, 2, 1};
+		assertEquals(1, InversionCount.getInversionCountFromArray(a));
+	}
+
+	@Test
+	void testInversionCount4() {
+		int[] a = {2, 1, 1};
+		assertEquals(2, InversionCount.getInversionCountFromArray(a));
+	}
+
+	@Test
+	void testMergeSort0() {
+		int[] a = {2, 3, 9, 2, 9};
+		int[] b = {2, 2, 3, 9, 9};
+		assertArrayEquals(b, InversionCount.mergeSortArray(a));
+	}
+
+	@Test
+	void testMergeSort1() {
+		int[] a = {5, 4, 3, 2, 1};
+		int[] b = {1, 2, 3, 4, 5};
+		assertArrayEquals(b, InversionCount.mergeSortArray(a));
+	}
+}