这是indexloc提供的服务，不要输入任何密码
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BAEL-8739 Count inversions in an array implementation #17952

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Nov 25, 2024
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BAEL-8739 Count inversions in an array implementation #17952

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89 changes: 89 additions & 0 deletions .../src/test/java/com/baeldung/algorithms/countinginversions/CountingInversionsUnitTest.java
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package com.baeldung.algorithms.countinginversions;

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

class CountingInversionsUnitTest {

@Test
void givenArray_whenCountingInversions_thenReturnCorrectCount() {
int[] input = {3, 1, 2};
int expectedInversions = 2;

int actualInversions = countInversionsBruteForce(input);

assertEquals(expectedInversions, actualInversions);
}

int countInversionsBruteForce(int[] array) {
int inversionCount = 0;
for (int i = 0; i < array.length - 1; i++) {
for (int j = i + 1; j < array.length; j++) {
if (array[i] > array[j]) {
inversionCount++;
}
}
}
return inversionCount;
}

@Test
void givenArray_whenCountingInversionsWithOptimizedMethod_thenReturnCorrectCount() {
int[] inputArray = {3, 1, 2};
int expectedInversions = 2;

int actualInversions = countInversionsDivideAndConquer(inputArray);

assertEquals(expectedInversions, actualInversions);
}

int countInversionsDivideAndConquer(int[] array) {
if (array == null || array.length <= 1) {
return 0;
}
return mergeSortAndCount(array, 0, array.length - 1);
}

int mergeSortAndCount(int[] array, int left, int right) {
if (left >= right) {
return 0;
}

int mid = left + (right - left) / 2;
int inversions = mergeSortAndCount(array, left, mid) + mergeSortAndCount(array, mid + 1, right);

inversions += mergeAndCount(array, left, mid, right);
return inversions;
}

int mergeAndCount(int[] array, int left, int mid, int right) {
int[] leftArray = new int[mid - left + 1];
int[] rightArray = new int[right - mid];

System.arraycopy(array, left, leftArray, 0, mid - left + 1);
System.arraycopy(array, mid + 1, rightArray, 0, right - mid);

int i = 0, j = 0, k = left, inversions = 0;

while (i < leftArray.length && j < rightArray.length) {
if (leftArray[i] <= rightArray[j]) {
array[k++] = leftArray[i++];
} else {
array[k++] = rightArray[j++];
inversions += leftArray.length - i;
}
}

while (i < leftArray.length) {
array[k++] = leftArray[i++];
}

while (j < rightArray.length) {
array[k++] = rightArray[j++];
}

return inversions;
}

}