<p>Given an integer array <code>nums</code>, return <em>the number of <strong>reverse pairs</strong> in the array</em>.</p>

<p>A <strong>reverse pair</strong> is a pair <code>(i, j)</code> where:</p>

<ul>
	<li><code>0 &lt;= i &lt; j &lt; nums.length</code> and</li>
	<li><code>nums[i] &gt; 2 * nums[j]</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [1,3,2,3,1]
<strong>Output:</strong> 2
<strong>Explanation:</strong> The reverse pairs are:
(1, 4) --&gt; nums[1] = 3, nums[4] = 1, 3 &gt; 2 * 1
(3, 4) --&gt; nums[3] = 3, nums[4] = 1, 3 &gt; 2 * 1
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,4,3,5,1]
<strong>Output:</strong> 3
<strong>Explanation:</strong> The reverse pairs are:
(1, 4) --&gt; nums[1] = 4, nums[4] = 1, 4 &gt; 2 * 1
(2, 4) --&gt; nums[2] = 3, nums[4] = 1, 3 &gt; 2 * 1
(3, 4) --&gt; nums[3] = 5, nums[4] = 1, 5 &gt; 2 * 1
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>-2<sup>31</sup> &lt;= nums[i] &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

const auto init = [](){
       std::cin.tie(nullptr);
       std::cout.tie(nullptr);
       std::ios_base::sync_with_stdio(false);
       return nullptr;
}();

class Solution {
public:

      int  count;


    void cntpair(vector<long long>& temp1,vector<long long>& temp2){
      int p1=0;
      for(int i=0;i<temp2.size()-1;i++){
            while(p1<temp1.size()-1&&temp2[i]!=LONG_LONG_MAX&&temp1[p1]<=2*temp2[i]){
                p1++;
            }
        
            this->count+=temp1.size()-1-p1;
      }

    }

    void merge(vector<int>& arr, int l, int m, int r)
    {
      vector<long long > temp1(m-l+2,INT_MAX);
        vector<long long> temp2(r-m+1,INT_MAX);
        
        for(int i=l;i<=m;i++){
            temp1[i-l]=arr[i];
        }
        for(int j=m+1;j<=r;j++ ){
            temp2[j-m-1]=arr[j];
        }
        temp1[m+1-l]=LONG_LONG_MAX;
        temp2[r-m]=LONG_LONG_MAX;
         int p1=0;
        int p2=0;
                        cntpair(temp1,temp2);

        for(int i=l;i<=r;i++){
            
            if(temp1[p1]<temp2[p2]){
                arr[i]=temp1[p1];
                p1++;
            }
            else{
                arr[i]=temp2[p2];
                p2++;
            }
        }

        
        
    }
    public:
    void mergeSort(vector<int>& arr, int l, int r)
    {
        if(l>=r){
            return;
        }
        int mid=(l+r)>>1;
        
        mergeSort(arr,l,mid);
        mergeSort(arr,mid+1,r);
        merge(arr,l,mid,r);
        
    }
   



    int reversePairs(vector<int>& nums) {
        this->count=0;
        mergeSort(nums,0,nums.size()-1);
        return this->count;
    }
};
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