<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;
    void mergeSort(vector<int> &vec, int start, int end){
        int i, j, k;
        int mid = start + (end-start)/2;
        int n1 = mid - start + 1;
        int n2 = end - mid;

        // Create temporary vectors
        vector<int> leftVec(n1), rightVec(n2);

        // Copy data to temporary vectors
        for (i = 0; i < n1; i++)
            leftVec[i] = vec[start + i];
        for (j = 0; j < n2; j++)
            rightVec[j] = vec[mid + 1 + j];

        // Merge the temporary vectors back into vec[left..right]
        i = 0;
        j = 0;

        while(i<n1 && j<n2){
            long long int comp=(2*(long long int)rightVec[j]);
            if(leftVec[i]>comp){
                ans+=(n1-i);
                j++;
            }
            else{
                i++;
            }
        }

        i = 0;
        j = 0;
        k = start;
        while (i < n1 && j < n2) {
            if (leftVec[i] <= rightVec[j]) {
                vec[k] = leftVec[i];
                i++;
            } else {
                vec[k] = rightVec[j];
                // if(leftVec[i]>2*rightVec[j]) ans+=(n1-i);
                j++;
            }
            k++;
        }

        // Copy the remaining elements of leftVec[], if any
        while (i < n1) {
            vec[k] = leftVec[i];
            i++;
            k++;
        }

        // Copy the remaining elements of rightVec[], if any
        while (j < n2) {
            vec[k] = rightVec[j];
            j++;
            k++;
        }
    }

    void merge(vector<int> &nums, int start, int end){
        if(start>=end) return;
        int mid=start+(end-start)/2;
        merge(nums,start,mid);
        merge(nums,mid+1,end);
        mergeSort(nums,start,end);
    }

    int reversePairs(vector<int>& nums) {
        int n=nums.size();
        ans=0;
        merge(nums,0,n-1);
        for(auto i:nums){
            cout<<i<<" ";
        }
        return ans;
    }
};
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