inversion-of-array.cpp

int cnt = 0;

void merge(vector<int> &arr, int low, int mid, int high) {
    vector<int> temp; // temporary array
    int left = low;      // starting index of left half of arr
    int right = mid + 1;   // starting index of right half of arr
    while (left <= mid && right <= high) {
        if (arr[left] <= arr[right]) {
            temp.push_back(arr[left]);
            left++;
        }

        // right is smaller
        else {
            temp.push_back(arr[right]);
            cnt += (mid - left + 1);  // Corrected: this line was already correct, just highlighting the addition to `cnt`
            right++;
        }
    }

    // if elements on the left half are still left //

    while (left <= mid) {
        temp.push_back(arr[left]);
        left++;
    }
    //  if elements on the right half are still left //
    while (right <= high) {
        temp.push_back(arr[right]);
        right++;
    }
    // transferring all elements from temporary to arr //
    for (int i = low; i <= high; i++) {
        arr[i] = temp[i - low];
    }
}

void mergeSort(vector<int> &arr, int low, int high) {
    if (low >= high) return;
    int mid = (low + high) / 2 ;
    mergeSort(arr, low, mid);  // left half
    mergeSort(arr, mid + 1, high); // right half
    merge(arr, low, mid, high);  // merging sorted halves
}

int numberOfInversions(vector<int> &a, int n) {
    cnt = 0;  // Correction: Reset `cnt` before counting inversions
    mergeSort(a, 0, n - 1);
    return cnt;
}