<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 divide(vector<int>& nums,int l,int r){
        if(l==r){
            return 0;
        }
        int mid = l+(r-l)/2;
        int cnt1 = divide(nums,l,mid);
        int cnt2 = divide(nums,mid+1,r);
        int p1=l,p2=mid+1;
        int cnt=0;
        vector<int> res;
        while(p1<=mid && p2<=r){
            if((nums[p1]*1LL)<=2LL*nums[p2]){
                res.push_back(nums[p1]);
                p1++;
            }else{
                cnt+=mid-p1+1;
                res.push_back(nums[p2]);
                p2++;
            }
        }
        while(p1<=mid){
            res.push_back(nums[p1++]);
        }
        while(p2<=r){
             res.push_back(nums[p2++]);
        }
        sort(res.begin(),res.end());
        for(int i=l;i<=r;i++){
            nums[i]=res[i-l];
        }
        return cnt+cnt1+cnt2;
    }
    int reversePairs(vector<int>& nums) {
        return divide(nums,0,nums.size()-1);
    }
};
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