Programación Competitiva

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

long long int _mergeSort(int arr[], int temp[], int left, int right);
long long int merge(int arr[], int temp[], int left, int mid, int right);

/* This function sorts the input array and returns the
   number of inversions in the array */
long long int mergeSort(int arr[], int array_size)
{
    int* temp = (int*)malloc(sizeof(int) * array_size);
    return _mergeSort(arr, temp, 0, array_size - 1);
}

/* An auxiliary recursive function that sorts the input array and
  returns the number of inversions in the array. */
long long int _mergeSort(int arr[], int temp[], int left, int right)
{
    long long int mid, inv_count = 0;
    if (right > left) {
        /* Divide the array into two parts and call _mergeSortAndCountInv()
       for each of the parts */
        mid = (right + left) / 2;

        /* Inversion count will be sum of inversions in left-part, right-part
      and number of inversions in merging */
        inv_count = _mergeSort(arr, temp, left, mid);
        inv_count += _mergeSort(arr, temp, mid + 1, right);

        /*Merge the two parts*/
        inv_count += merge(arr, temp, left, mid + 1, right);
    }
    return inv_count;
}

/* This funt merges two sorted arrays and returns inversion count in
   the arrays.*/
long long int merge(int arr[], int temp[], int left, int mid, int right)
{
    int i, j, k;
    long long int inv_count = 0;

    i = left; /* i is index for left subarray*/
    j = mid; /* j is index for right subarray*/
    k = left; /* k is index for resultant merged subarray*/
    while ((i <= mid - 1) && (j <= right)) {
        if (arr[i] <= arr[j]) {
            temp[k++] = arr[i++];
        }
        else {
            temp[k++] = arr[j++];

            /*this is tricky -- see above explanation/diagram for merge()*/
            inv_count = inv_count + (mid - i);
        }
    }

    /* Copy the remaining elements of left subarray
   (if there are any) to temp*/
    while (i <= mid - 1)
        temp[k++] = arr[i++];

    /* Copy the remaining elements of right subarray
   (if there are any) to temp*/
    while (j <= right)
        temp[k++] = arr[j++];

    /*Copy back the merged elements to original array*/
    for (i = left; i <= right; i++)
        arr[i] = temp[i];

    return inv_count;
}

int main()
{
    int n;
    while(scanf("%d",&n)==1 && n!=0)
    {
        int arr[n];
        for(int i=0; i<n;i++) scanf("%d",&arr[i]);
        printf("%lld\n", mergeSort(arr, n));
    }
    return 0;
}