count_inversion_merge_sort.cpp

#include<bits/stdc++.h>
using namespace std;

long long merge(int arr[], int l, int mid, int r) {
    long long 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+i+1];
    }

    int i=0, j=0, k=l;
    while(i<n1 and j<n2) {
        if(a[i] <= b[j]) {
            arr[k] = a[i];
            k++; i++;
        } else
        {
            arr[k] = b[j];
            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;
}

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

int main() {

    int n;
    cin>>n;

    int arr[n];

    for(int i=0; i<n; i++) {
        cin>>arr[i];
    }

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