MergeSort 구현

practice/mergesort/.gitattributes

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+#
+# https://help.github.com/articles/dealing-with-line-endings/
+#
+# These are explicitly windows files and should use crlf
+*.bat           text eol=crlf
+

practice/mergesort/.gitignore

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+# Ignore Gradle project-specific cache directory
+.gradle
+
+# Ignore Gradle build output directory
+build

practice/mergesort/app/build.gradle

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+/*
+ * This file was generated by the Gradle 'init' task.
+ *
+ * This generated file contains a sample Java application project to get you started.
+ * For more details take a look at the 'Building Java & JVM projects' chapter in the Gradle
+ * User Manual available at https://docs.gradle.org/7.0/userguide/building_java_projects.html
+ */
+
+plugins {
+    // Apply the application plugin to add support for building a CLI application in Java.
+    id 'application'
+}
+
+repositories {
+    // Use Maven Central for resolving dependencies.
+    mavenCentral()
+}
+
+dependencies {
+    // Use JUnit test framework.
+    testImplementation 'junit:junit:4.13.1'
+
+    // This dependency is used by the application.
+    implementation 'com.google.guava:guava:30.0-jre'
+}
+
+application {
+    // Define the main class for the application.
+    mainClass = 'mergesort.App'
+}

practice/mergesort/app/src/main/java/mergesort/App.java

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+/*
+ * This Java source file was generated by the Gradle 'init' task.
+ */
+package mergesort;
+
+public class App {
+    public static void main(String[] args) {
+        int[] array = {3,9,4,7,5,0,1,6,8,2};
+
+        int[] resultArray = new App().solution(array);
+
+        System.out.print("resultArray : ");
+        for (int a: resultArray) {
+            System.out.print(a + ", ");
+        }
+        System.out.println();
+    }
+
+    public void mergeSort(int[] arr, int[] tmp, int start, int end) {
+        // start = end 같을 경우, 원소가 한개이므로, sort할 필요 없다.
+        // 따라서, start < end 인 경우에만, sort!
+        if(start < end) {
+            int mid = (start + end) / 2;
+
+            // 왼쪽 정렬
+            mergeSort(arr, tmp, start, mid);
+
+            // 오른쪼 정렬
+            mergeSort(arr, tmp, mid + 1, end);
+
+            // 이미 정렬된 배열 병합
+            merge(arr, tmp, start, mid, end);
+        }
+    }
+
+    private void merge(int[] arr, int[] tmp, int start, int mid, int end) {
+        // 정렬된 배열을 임시저장소에 필요한 만큼 복사 --> why? 비교할 때 기존 값을 보존하기 위해,
+        for (int i = start; i <= end; i++) {
+            tmp[i] = arr[i];
+        }
+
+        int part1FirstIndex = start;
+        int part2FirstIndex = mid + 1;
+
+        int resultArrayIndex = start;
+
+        while (part1FirstIndex <= mid && part2FirstIndex <= end) {
+            if(tmp[part1FirstIndex] <= tmp[part2FirstIndex]) {
+                arr[resultArrayIndex] = tmp[part1FirstIndex];
+                part1FirstIndex++;
+            } else {
+                arr[resultArrayIndex] = tmp[part2FirstIndex];
+                part2FirstIndex++;
+            }
+            resultArrayIndex++;
+        }
+
+        // part1에 데이터가 남아 있고 part2는 비어있는 경우(예 : 5,7,9 / 1, 2) : for문 돌려주기
+        // cf. part1에 비어있고, part2가 남아 있는 경우에는 for문이 필요 없다. 왜냐하면 이미 정렬되어 있기 때문에
+        for (int i = 0; i <= mid - part1FirstIndex; i++) {
+            arr[resultArrayIndex + i] = tmp[part1FirstIndex + i];
+        }
+    }
+
+    public int[] solution(int[] arr) {
+        int[] tmp = new int[arr.length];
+
+        mergeSort(arr, tmp, 0, arr.length - 1);
+
+        return arr;
+    }
+}

practice/mergesort/app/src/test/java/mergesort/AppTest.java

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+/*
+ * This Java source file was generated by the Gradle 'init' task.
+ */
+package mergesort;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class AppTest {
+
+    @Test
+    public void testMergeSort() {
+        // given
+        int[] array = {3,9,4,7,5,0,1,6,8,2};
+
+        // when
+        int[] resultArray = new App().solution(array);
+
+        // then
+        int[] expectedArray = {0,1,2,3,4,5,6,7,8,9};
+
+        assertArrayEquals(expectedArray, resultArray);
+    }
+}

practice/mergesort/gradle/wrapper/gradle-wrapper.properties

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.0-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists