GeeksforGeeks-Solution/src/Count Inversions/Code/solution.cpp at main · withaarzoo/GeeksforGeeks-Solution · GitHub

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class Solution
{
public:
    long long mergeAndCount(vector<int> &arr, int left, int mid, int right)
    {
        vector<int> temp;
        int i = left;
        int j = mid + 1;
        long long invCount = 0;

        while (i <= mid && j <= right)
        {
            if (arr[i] <= arr[j])
            {
                temp.push_back(arr[i]);
                i++;
            }
            else
            {
                temp.push_back(arr[j]);
                invCount += (mid - i + 1);
                j++;
            }
        }

        while (i <= mid)
        {
            temp.push_back(arr[i]);
            i++;
        }

        while (j <= right)
        {
            temp.push_back(arr[j]);
            j++;
        }

        for (int k = left; k <= right; k++)
        {
            arr[k] = temp[k - left];
        }

        return invCount;
    }

    long long mergeSort(vector<int> &arr, int left, int right)
    {
        long long invCount = 0;
        if (left < right)
        {
            int mid = left + (right - left) / 2;

            invCount += mergeSort(arr, left, mid);
            invCount += mergeSort(arr, mid + 1, right);
            invCount += mergeAndCount(arr, left, mid, right);
        }
        return invCount;
    }

    long long inversionCount(vector<int> &arr)
    {
        return mergeSort(arr, 0, arr.size() - 1);
    }
};