<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 total=0;
    void merge(vector<int>&nums,int low,int mid,int high){
        int left=low;
        int right=mid+1;
        int cnt=0;
        while(left<=mid && right <= high){
            if(nums[left] > (2*(long)nums[right])){
                cnt+=(mid-left+1);
                right++;
            }
            else left++;
        }
        left=low;
        right=mid+1;
        vector<int>temp(high-low+1);
        int k=0;
        while(left<=mid && right <=high){
            if(nums[left]<=nums[right]){
                temp[k++]=nums[left++];
            }
            else{
                temp[k++]=nums[right++];
            }
        }
        while(left<=mid){
            temp[k++]=nums[left++];
        }
        while(right<=high){
            temp[k++]=nums[right++];
        }

        for(int i=low;i<=high;i++){
            nums[i]=temp[i-low];
        }

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