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Inversion Count for an array indicates
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package com.hrishikeshmishra.practices.sort; | |
import java.util.Arrays; | |
import static com.hrishikeshmishra.practices.sort.InversionsCountInArray.getCountByMergeSort; | |
/** | |
* Problem: | |
* Inversion Count for an array indicates – | |
* how far (or close) the array is from being sorted. | |
* | |
* @author hrishikesh.mishra | |
* @link http://hrishikeshmishra.com/inversion-count/ | |
*/ | |
public class InversionsCountInArray { | |
public static long getCountByBruteForce(int[] a) { | |
long inversionCount = 0; | |
for (int i = 0; i < a.length - 1; i++) { | |
for (int j = i; j < a.length; j++) { | |
if (a[i] > a[j]) { | |
inversionCount++; | |
} | |
} | |
} | |
return inversionCount; | |
} | |
public static long getCountByMergeSort(int[] a) { | |
return mergeSort(a, 0, a.length - 1); | |
} | |
public static long mergeSort(int[] a, int left, int right) { | |
if (right > left) { | |
int mid = (left + right) / 2; | |
long inversionCount = mergeSort(a, left, mid); | |
inversionCount += mergeSort(a, mid + 1, right); | |
inversionCount += merge(a, left, mid + 1, right, new int[a.length]); | |
return inversionCount; | |
} | |
return 0; | |
} | |
private static long merge(int[] a, int left, int mid, int right, int[] temp) { | |
int i = left; | |
int j = mid; | |
int counter = left; | |
long inversionCount = 0; | |
while (i < mid && j <= right) { | |
if (a[i] <= a[j]) { | |
temp[counter++] = a[i++]; | |
} else { | |
temp[counter++] = a[j++]; | |
inversionCount += (mid - i); | |
} | |
} | |
while (i < mid) { | |
temp[counter++] = a[i++]; | |
} | |
while (j <= right) { | |
temp[counter++] = a[j++]; | |
} | |
for (int c = left; c <= right; c++) { | |
a[c] = temp[c]; | |
} | |
return inversionCount; | |
} | |
} | |
class InversionsCountInArrayTest { | |
public static void main(String[] args) { | |
int[] array1 = {1, 1, 1, 2, 2}; | |
int[] array2 = {2, 1, 3, 1, 2}; | |
int[] array3 = {1, 1, 1, 2, 2}; | |
int[] array4 = {2, 1, 3, 1, 2}; | |
getCountByMergeSort(array2); | |
System.out.println("Array: " + Arrays.toString(array1)); | |
System.out.println("Inversion Count by Brute Force: " + InversionsCountInArray.getCountByBruteForce(array1)); | |
System.out.println("Inversion Count by Merge Sort: " + InversionsCountInArray.getCountByMergeSort(array1)); | |
System.out.println(); | |
System.out.println("Array: " + Arrays.toString(array2)); | |
System.out.println("Inversion Count by Brute Force: " + InversionsCountInArray.getCountByBruteForce(array2)); | |
System.out.println("Inversion Count by Merge Sort: " + InversionsCountInArray.getCountByMergeSort(array2)); | |
System.out.println(); | |
System.out.println("Array: " + Arrays.toString(array3)); | |
System.out.println("Inversion Count by Brute Force: " + InversionsCountInArray.getCountByBruteForce(array3)); | |
System.out.println("Inversion Count by Merge Sort: " + InversionsCountInArray.getCountByMergeSort(array3)); | |
System.out.println(); | |
System.out.println("Array: " + Arrays.toString(array4)); | |
System.out.println("Inversion Count by Brute Force: " + InversionsCountInArray.getCountByBruteForce(array4)); | |
System.out.println("Inversion Count by Merge Sort: " + InversionsCountInArray.getCountByMergeSort(array4)); | |
System.out.println(); | |
} | |
} |
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