<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 ans = 0;
    void merge(vector<int>& nums, int s, int e, int mid){
        int len1 = mid - s + 1;
        int len2 = e - mid;
        int *first = new int [len1];
        int *second = new int [len2];
        int k = s;
        for(int i=0;i<len1;i++){
            first[i] = nums[k++];
        }
        k = mid + 1;
        for(int i=0;i<len2;i++){
            second[i] = nums[k++];
        }
        int j = 0, count = 0;
        for(int i = 0;i<len1;i++){
            while(j  < len2 && first[i] > (long)2* second[j]){
                 j++;
            }
            count += j;
        }
        ans += count;
        
        int index1 = 0, index2 = 0;
        k = s;
        while(index1 < len1 && index2 < len2){
            if(first[index1] <= second[index2]){
                nums[k++] = first[index1++];
            }
            else{
                nums[k++] = second[index2++];
            }
        }

        while(index1 < len1){
            nums[k++] = first[index1++];
        }
        while(index2 < len2){
            nums[k++] = second[index2++];
        }

        delete []first;
        delete []second;
    }
    void mergesort(vector<int>& nums, int s, int e, int n){
        if(s >= e)return;
        int mid = s+(e-s)/2;
        mergesort(nums, s, mid, n);
        mergesort(nums, mid+1, e, n);
        merge(nums, s, e, mid);
    }
    int reversePairs(vector<int>& nums) {
        int n = nums.size();
        mergesort(nums, 0, n-1, n);
        return ans;
    }
};
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