#include<bits/stdc++.h>

​

using namespace std;

int Merge(vector < int > & nums, int low, int mid, int high) {

  int total = 0;

  int j = mid + 1;

  for (int i = low; i <= mid; i++) {

    while (j <= high && nums[i] > 2 LL * nums[j]) {

      j++;

    }

    total += (j - (mid + 1));

  }

​

  vector < int > t;

  int left = low, right = mid + 1;

​

  while (left <= mid && right <= high) {

​

    if (nums[left] <= nums[right]) {

      t.push_back(nums[left++]);

    } else {

      t.push_back(nums[right++]);

    }

  }

​

  while (left <= mid) {

    t.push_back(nums[left++]);

  }

  while (right <= high) {

    t.push_back(nums[right++]);

  }

​

  for (int i = low; i <= high; i++) {

    nums[i] = t[i - low];

  }

  return total;

}

int MergeSort(vector < int > & nums, int low, int high) {

​

  if (low >= high) return 0;

  int mid = (low + high) / 2;

  int inv = MergeSort(nums, low, mid);

  inv += MergeSort(nums, mid + 1, high);

  inv += Merge(nums, low, mid, high);

  return inv;

}

int reversePairs(vector < int > & arr) {

  return MergeSort(arr, 0, arr.size() - 1);

}

​

int main() {

  vector<int> arr{1,3,2,3,1};

  cout << "The Total Reverse Pairs are " << reversePairs(arr);

}