<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 cnt=0;
    void merge(vector<int>& arr,int left,long mid,int right)
    {
        vector<int>temp;
        int p=left;
        long q=mid;
        while(p<mid && q<=right)
        {
            if(arr[p]>(long)arr[q]*2)
            {
                temp.push_back(arr[q]);
                q++;
                cnt+=mid-p;
            }else
            {
                temp.push_back(arr[p]);
                p++;
            }
        }
        while(p<mid)
        {
             temp.push_back(arr[p]);
             p++;
        }
        while(q<=right)
        {
            temp.push_back(arr[q]);
            q++;
        }
        sort(temp.begin(),temp.end());
        for(int i=left;i<=right;i++)
        arr[i]=temp[i-left];
    }
    void msort(vector<int>& arr,int left,int right)
    {
        // cout<<left<<" "<< right<<endl;
        if(left>=right)
        return;
        long mid=(right+left)/2;

        msort(arr,left,mid);
        msort(arr,mid+1,right);
        merge(arr,left,mid+1,right);
    }
    int reversePairs(vector<int>& nums) {
        cnt=0;
        msort(nums,0,nums.size()-1);
        return cnt;
    }
};
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