<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 {
    int mergesort(vector<int> &nums,int st,int en){
        if(st==en) return 0;
        int mid=(st+en)/2;
        int res=0;
        res+=mergesort(nums,st,mid);
        res+=mergesort(nums,mid+1,en);
        int st1=st;
        int st2=mid+1;
        int st3=mid+1;
        vector<int> v;
        while(st1<=mid){
            while(st2<=en && (nums[st1]>2L*nums[st2])) st2++;
            res+=st2-mid-1;
            while(st3<=en && (nums[st1]>=nums[st3])){
                v.push_back(nums[st3++]);
            }
            v.push_back(nums[st1++]);
        }
        for(int i=0; i<v.size(); i++){
            nums[st+i]=v[i];
        }
        return res;
    }
public:
    int reversePairs(vector<int>& nums) {
        int st=0,en=nums.size()-1;
        return mergesort(nums,st,en);
    }
};
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