<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>

#define ll long long int;
class Solution {
public:
void mergeSort(vector<int>&arr, int l, int r, long long &inv) {
    if (l >= r) return;
    
    int mid = l + (r - l) / 2;
    mergeSort(arr, l, mid, inv);
    mergeSort(arr, mid + 1, r, inv);
    merge(arr, l, mid, r, inv);
}

void count_inv(vector<int>& arr, int l, int m, int r, long long &inv){
    int x = l;
    int y = m + 1;   
    long long ct =0;

    while(x<=m&&y<=r){
        if(1ll*arr[x]>2ll*arr[y]){
            inv+=(m-x+1);
            y++;
        }
        else x++;

        
    }
  
    
}
void merge(vector<int>& arr, int l, int m, int r, long long &inv) {
    int x = l;
    int y = m + 1;

    
    count_inv(arr,l,m,r,inv);
    vector<long long> temp(r - l + 1);  // This line caused a compilation error
    
    int k = 0;
    while (x <= m && y <= r) {
        if (arr[x] < arr[y]) {
           

            temp[k] = arr[x];
            x++;
        } else {
            int p = x;
            temp[k] = arr[y];
            y++;
            
        }
        k++;
    }
    
    while (x <= m) {
        temp[k] = arr[x];
        k++;
        x++;
    }
    
    while (y <= r) {
        temp[k] = arr[y];
        k++;
        y++;
    }
    
    // Copy sorted elements from temp back to arr
    for (int i = 0; i < temp.size(); i++) {
        arr[l] = temp[i];
        l++;
    }
}

    int reversePairs(vector<int>& nums) {
        int n = nums.size();
        long long inv=0;
        mergeSort(nums,0,n-1,inv);
        return (int)inv;
        
    }
};










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