<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:

   int merge(vector<int>&v,int st,int end,int mid){
    long long i=st,j=mid+1,ans=0;
    vector<int>temp;

    while(i<=mid && j<=end){
       if(v[i]>2LL*v[j]){
        temp.push_back(v[j]);
        ans+=(mid-i+1);
        j++;
       }
       else{
        temp.push_back(v[i]);
        i++;
       }
    }

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

    while(j<=end){
         temp.push_back(v[j]);
        j++;
    }

    sort(temp.begin(),temp.end());
    j=0;
    for(int k=st;k<=end;k++) v[k]=temp[j++];
    return ans;


   }


    int fun(vector<int>&v,int st,int end){
        if(end<=st) return 0;
        int mid=(st+end)/2;
        long long a=fun(v,st,mid);
        long long b=fun(v,mid+1,end);
        long long c=merge(v,st,end,mid);
        return a+b+c;
    }

    int reversePairs(vector<int>&v) {
        int n=v.size();
        return fun(v,0,n-1);
    }
};
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