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CountNumberOfInversions.java
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CountNumberOfInversions.java
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package DivideAndConquer;
import java.util.Arrays;
public class CountNumberOfInversions {
static int findInversionCount(int arr[], int left, int right) {
int inversionCount = 0;
if (left < right) {
// Find the middle point
int mid = left + (right - left) / 2;
// Sort first and second halves and store the inversion count
inversionCount += findInversionCount(arr, left, mid);
inversionCount += findInversionCount(arr, mid + 1, right);
// Merge the sorted halves
inversionCount += merge(arr, left, mid, right);
}
return inversionCount;
}
static int merge(int arr[], int left, int mid, int right) {
int inversionCount = 0;
// Left subarray
int[] leftHalf = Arrays.copyOfRange(arr, left, mid + 1);
// Right subarray
int[] rightHalf = Arrays.copyOfRange(arr, mid + 1, right + 1);
int i = 0, j = 0, k = left;
while (i < leftHalf.length && j < rightHalf.length) {
if (leftHalf[i] <= rightHalf[j])
arr[k++] = leftHalf[i++];
else {
arr[k++] = rightHalf[j++];
inversionCount += (mid + 1) - (left + i);
}
}
while(i < leftHalf.length)
arr[k++] = leftHalf[i++];
while(j < rightHalf.length)
arr[k++]=rightHalf[j++];
return inversionCount;
}
public static void main(String[] args) {
int[] arr = new int[]{5, 3, 2, 4, 1};
System.out.println(findInversionCount(arr, 0 , arr.length - 1));
}
}