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inversion_count.cpp
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inversion_count.cpp
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#include <bits/stdc++.h>
using namespace std;
int merge(int arr[], int low, int mid, int high)
{
int i = low, j = mid + 1, k = low;
int b[high];
int count=0;
while (i <= mid && j <= high)
{
if (arr[i] <= arr[j])
{
b[k] = arr[i];
i++;
k++;
}
else
{
b[k] = arr[j];
j++;
k++;
count+=mid+1-i;
}
}
if (i > mid)
{
while (j<=high)
{
b[k]=arr[j];
j++;
k++;
}
}
else{
while (i<=mid)
{
b[k]=arr[i];
i++;
k++;
}
}
for(k=low;k<=high;k++)
arr[k]=b[k];
return count;
}
int mergesort(int arr[], int low, int high)
{
int mid;
int count = 0;
if (high > low)
{
mid = (high + low) / 2;
count += mergesort(arr, low, mid);
count += mergesort(arr, mid + 1, high);
count += merge(arr, low, mid, high);
}
return count;
}
int main()
{
int n;
cout << "enter array length=";
cin >> n;
int arr[n];
cout << "\n enter array to be sorted=";
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
cout << mergesort(arr, 0, n);
cout << endl;
return 0;
}