C :

#include <stdio.h>
#include <string.h>
typedef long long LL;
int num[100005];
int backup[100005];
LL count_inversions(int a[], int b[], int low, int high)
{
   int i, j, mid = (low + high) / 2;
   LL tot = 0;
   if(low == high)
      return 0;
   tot += count_inversions(b, a, low, mid) + count_inversions(b, a, mid + 1, high);
   int k = low;
   for(i = low, j = mid + 1; i <= mid && j <= high;)
      if(b[i] > b[j]) {
         a[k++] = b[j++];
         tot += (mid - i + 1);
      } else {
         a[k++] = b[i++];
      }
   while(i <= mid) {
      a[k++] = b[i++];
   }
   while(j <= high) {
      a[k++] = b[j++];
   }
   return tot;
}
int main()
{
   int i, n;
   scanf("%d", &n);
   for(i = 0; i < n; i++)
      scanf("%d", &num[i]);
   memcpy(backup, num, sizeof num);
   printf("%lld\n", count_inversions(num, backup, 0, n - 1));

   return 0;
}

C++ :

#include <iostream>
using namespace std;
int i,j,n,a[100001],b[100001];
long long k;
void deseq(int l,int r)
{
    int i,j,x,q,p;
    if (r-l>1)
    {
        x=(r+l)/2;
        q=l;p=x;i=l;
        deseq(l,x);
        deseq(x,r);
        while (i<r)
        {
            if (p>=r || (q<x && a[q]<=a[p])) {b[i++]=a[q++];}
            else {b[i++]=a[p++];k+=x-q;}
        }
        for (i=l;i<r;i++)
        {
            a[i]=b[i];
        }
    }
    return;
}
int main()
{
    cin>>n;
    if (n<=0) return 0;
    for (i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
    }
    deseq(1,n+1);
    cout<<k;
    return 0;
}