<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 {
    void merge(vector<int>&nums,int low,int mid,int high){
        int start=low;
        int right=mid+1;
        vector<int>temp;
        while(low<=mid and right<=high){
            if(nums[low]<nums[right]){
             temp.push_back(nums[low]);
             low++;
            }
            else if(nums[low]==nums[right]){
                temp.push_back(nums[low]);
                temp.push_back(nums[low]);
                low++;
                right++;
            }
            else{
                temp.push_back(nums[right]);
                right++;
            }
        }
        while(low<=mid){
            temp.push_back(nums[low]);
            low++;
        }
        while(right<=high){
            temp.push_back(nums[right]);
            right++;
        }
        for(int i=start;i<=high;i++){
            nums[i]=temp[i-start];
        }
    }
    void countpair(vector<int>&nums,int low,int mid,int high,int &cnt){
            int start=low;
            int right=mid+1;
            for(int i=start;i<=mid;i++){
                while(right<=high and 2LL*nums[right]<nums[i]){
                    right++;
                }
                cnt+=right-(mid+1);
            }

    }
    void mergesort(vector<int>&nums,int low,int high,int &cnt){
        if(low>=high) return;
        int mid=low+(high-low)/2;
        mergesort(nums,low,mid,cnt);
        mergesort(nums,mid+1,high,cnt);
        countpair(nums,low,mid,high,cnt);
        merge(nums,low,mid,high);
    }
public:
    int reversePairs(vector<int>& vec) {
     int cnt=0;
     mergesort(vec,0,vec.size()-1,cnt);
     return cnt;  
    }
};

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