KattisQns/froshweek.cpp at master · MuhammadHoze/KattisQns · GitHub

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//// C++ program to Count
//// Inversions in an array
//// using Merge Sort
//#include <bits/stdc++.h>
//using namespace std;
//
//
//int merge(vector <long long> &arr, long long temp[], int left, int mid, int right){
//	int i, j, k;
//	long long inv_count = 0;
//
//	i = left;
//	j = mid;
//	k = left;
//	while ((i <= mid - 1) && (j <= right)) {
//		if (arr[i] <= arr[j]) {
//			temp[k++] = arr[i++];
//		}
//		else {
//			temp[k++] = arr[j++];
//
//			inv_count = inv_count + (mid - i);
//		}
//	}
//
//	//This while loop is if the 1st array has remaining values
//	while (i <= mid - 1)
//		temp[k++] = arr[i++];
//
// 	//This while loop is if the 2nd array has remaining values
//	while (j <= right)
//		temp[k++] = arr[j++];
//
//	//We need to copy all element to original arrays
//	for (i = left; i <= right; i++)
//		arr[i] = temp[i];
//
//	return inv_count;
//}
//
//
///* An auxiliary recursive function that sorts the input array and
//returns the number of inversions in the array. */
//int _mergeSort(vector <long long> &arr, long long temp[], int left, int right)
//{
//	int mid, inv_count = 0;
//	if (right > left) {
//
//		//1. Divide
//		mid = (right + left) / 2;
//
//		// 2. Recursively divide the vector
//		inv_count = _mergeSort(arr, temp, left, mid); // divide the vector into a single element from start to mid
//		inv_count += _mergeSort(arr, temp, mid + 1, right); // divide the vector into a single element from mid+1 to end
//
//		// 3. Sort & merge the two parts
//		inv_count += merge(arr, temp, left, mid + 1, right);
//	}
//	return inv_count;
//}
//
//
//// sorts the input array and returns the number of inversions
//int mergeSort(vector <long long> &arr, long long array_size)
//{
//	long long temp[array_size];
//	return _mergeSort(arr, temp, 0, array_size - 1);
//}
//
//
//
//int main()
//{
//
//	long long n;
//	vector<long long> arr;
//
//	cin >> n;
//
//	for(int i=0; i<n; i++){
//		long long value;
//
//		cin >> value;
//		arr.push_back(value);
//	}
//
//	//int arr[100000];
//
//	//cin >> n;
//
//
////	for(int i=0; i<n; i++){
////		//long long value;
////
////		cin >> arr[i];
////		//number.push_back(value);
////	}
//
//	long long ans = mergeSort(arr, n);
//	cout << endl;
//	cout << ans;
//	return 0;
//}
//


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

int array1[1000005];
int array2[1000005];

void mergesort(int *array, int len, int offset)
{
    if (len > 1)
    {
        int *first = array1 + offset;
        if (first == array)
            first = array2 + offset;

        for (int i = 0; i < len; ++i)
            first[i] = array[i];

        int firstLength = len / 2;
        int secondStart = firstLength, secondLength = len - secondStart;
        int *second = first + secondStart;

        mergesort(first, firstLength, offset);
        mergesort(second, secondLength, offset + secondStart);

        int i = 0, j = 0;
        int pos = 0;

        int currentCost = count;

        while (i < firstLength && j < secondLength)
        {
            if (first[i] < second[j])
                array[pos++] = first[i++];
            else
            {
                array[pos++] = second[j++];
                count += firstLength - i;
            }
        }

        for (; i < firstLength; ++i, ++pos)
            array[pos] = first[i];
        for (; j < secondLength; ++j, ++pos)
            array[pos] = second[j];
    }
}

int main()
{
    int N;

    while (cin >> N)
    {
        for (int i = 0; i < N; ++i)
            cin >> array1[i];

        mergesort(array1, N, 0);

        cout << count << '\n';
        count = 0;
    }
}