<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>

class Solution {
public:
    void merge(vector<int>&arr,int low,int mid,int high){
        int left=low;
        int right=mid+1;
        int res=0;
        int k=0;
        vector<long long>temp(high-low+1);
        while(left<=mid&&right<=high){
            if(arr[left]<=arr[right]){
            temp[k++]=arr[left++];
            
}
else { temp[k++]=arr[right++];


}
            
        }
        while(left<=mid){
             temp[k++]=arr[left++];
            
        }
        while(right<=high){
 temp[k++]=arr[right++];
        }
        for(int i=0;i<temp.size();i++)
        {
            arr[i+low]=temp[i];
        }
    }
int cp(vector<int>& arr ,int low,int mid,int high){
  int right=mid+1;
    int ans=0;
    for(int i=low;i<=mid;i++){
        while(right<=high&&arr[i]>(long long)2*arr[right])
            right++;
            ans+=right-mid-1;
        
    }
    return ans;
}    int mergeSort(vector<int>&arr,int low,int high){
     int count=0;
        if(low>=high)
        return count;
    int mid=low+(high-low)/2;
    count+=mergeSort(arr,low,mid);
    count+=mergeSort(arr,mid+1,high);
        count+=cp(arr,low,mid,high);
    merge(arr,low,mid,high);
    return count;
    
    }
    int reversePairs(vector<int>& nums) {
        return mergeSort(nums,0,nums.size()-1);
    }
};
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