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P12.java
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P12.java
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import java.util.*;
import java.io.*;
public class P12 {
public static int merge(long[] arr, int left, int mid, int right) {
int inversion_count = 0;
long[] arr1 = Arrays.copyOfRange(arr, left, mid + 1);
long[] arr2 = Arrays.copyOfRange(arr, mid + 1, right + 1);
int n1 = arr1.length;
int n2 = arr2.length;
int i = 0, j = 0, k = left;
while (i < n1 && j < n2) {
if (arr1[i] <= arr2[j])
arr[k++] = arr1[i++];
else {
arr[k++] = arr2[j++];
inversion_count += (mid + 1 - left - i);
}
}
while (i < n1) {
arr[k++] = arr1[i++];
}
while (j < n2) {
arr[k++] = arr2[j++];
}
return inversion_count;
}
public static int mergeSort(long[] arr, int left, int right) {
int inversion_count = 0;
if (left < right) {
int mid = (left + right) >> 1;
inversion_count += mergeSort(arr, left, mid);
inversion_count += mergeSort(arr, mid + 1, right);
inversion_count += merge(arr, left, mid, right);
}
return inversion_count;
}
public static long getInversions(long arr[], int n) {
return mergeSort(arr, 0, n - 1);
}
}