<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 reverse(vector<int>&nums,int n1,vector<int>b,int n2){
        int i=0,j=0;
        int cnt=0;
        while(i<n1 && j<n2){
            long long val=(long long)2*(long long)b[j];
            if(nums[i]>val){
                cnt+=n1-i;
                j++;
            }
            else i++;
        }
        return cnt;
    }
    void merge(vector<int>&nums,int n1,vector<int>b,int n2){
        int i=n1-1;
        int j=n2-1;
        int k=(n1+n2)-1;
        while(j>=0){
            if(i>=0&&nums[i]>b[j]) nums[k--]=nums[i--];
            else nums[k--]=b[j--];
        }
    }
    int mergesort(vector<int>& nums,int n){
        if(n==1) return 0;
        int n1=n/2 ,n2=n-n/2;
        vector<int>b(n2);
        for(int i=0;i<n2;i++){
            b[i]=nums[i+n1];
        }
        int c=0;
        c+=mergesort(nums,n1);
        c+=mergesort(b,n2);
        c+=reverse(nums,n1,b,n2);
        merge(nums,n1,b,n2);
        b.clear();
        return c;
    }
    int reversePairs(vector<int>& nums) {
        int c=0;
        int n=nums.size();
        c=mergesort(nums,n);
        return c;
    }
};
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