<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:
    long long merge(long long l,long long mid,long long r,vector<long long> &v) {
        vector<long long> t(r-l+1);
        long long i=l,j=mid+1,k=0,res=0;
        while(j<=r) {
            while(i<=mid && v[i]<=2*v[j]) {
                i++;
            }
            res+=mid-i+1;
            j++;
        }
        i=l,j=mid+1;
        while(i<=mid && j<=r) {
            if(v[i]<v[j]) {
                t[k++]=v[i++];
            }
            else {
                t[k++]=v[j++];
            }
        }
        while(i<=mid) {
            t[k++]=v[i++];
        }
        while(j<=r) {
            t[k++]=v[j++];
        }
        for(long long ind=0;ind<r-l+1;ind++) {
            v[ind+l]=t[ind];
        }
        return res;
    }
    long long mergeSort(long long l,long long r,vector<long long> &v) {
        if(l>=r) {
            return 0;
        }
        long long mid=(l+r)/2;
        return mergeSort(l,mid,v)+mergeSort(mid+1,r,v)+merge(l,mid,r,v);
    }
    int reversePairs(vector<int> &nums) {
        long long n=nums.size();
        vector<long long> v;
        for(auto x:nums) {
            v.push_back(x);
        }
        return mergeSort(0,n-1,v);
    }
};
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