countInversion.cpp

#include <iostream>
using namespace std;

int merge(int arr[], int l, int mid , int r)
{
    int inv =0 ;
    int n1 = mid-l+1;
    int n2 = r-mid;

    int a[n1];
    int b[n2];

    for(int i=0; i<n1; i++)
    {
        a[i] = arr[l+i];
    }
    for(int i=0; i<n2; i++)
    {
        b[i] = arr[mid+1+i];
    }

    int i=0;
    int j=0;
     int k = l;

     while(i<n1 && j<n2)
     {
         if(a[i]<=b[j])
         {
             arr[k] = a[i];
             i++;
             k++;

         }
         else{
             arr[k] = b[j];
             inv = inv + (n1-i);
             k++;
             j++;

         }
     }
     while(i<n1)
     {
         arr[k] = a[i];
         k++;
         i++;
     }
     while(j<n2)
     {
         arr[k] = b[j];
         k++;
         j++;
     }
     return inv;
}

int mergeSort(int arr[], int l, int r)
{
    int inv = 0;
    if(l<r)
    {
        int mid = (l+r)/2;
        inv = inv + mergeSort(arr,l,mid);
        inv = inv + mergeSort(arr,mid+1, r);
        inv = inv + merge(arr,l,mid,r);

    }
    return inv;
}

int main(){

    int arr[] = {10,10,10};
    int n = sizeof(arr)/sizeof(arr[0]);

    cout<<mergeSort(arr,0,n-1);

    return 0;
}