<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 {
	int mergeSort(vector<int>& nums, int l, int r) {
        if(l < r) {
            int mid = l + (r - l) / 2;
            return mergeSort(nums, l, mid) +
                mergeSort(nums, mid + 1, r) +
                merge(nums, l, mid, r);
        }
        return 0;
    }

int merge(vector<int>& nums, int l, int mid, int r){
	int s1 = mid - l + 1;
	int s2 = r - mid;
	
	vector<int> lVec(s1);
	vector<int> rVec(s2);

	copy(nums, lVec, l, mid);
	copy(nums, rVec, mid + 1, r);

	int lInd = 0;
	int rInd = 0;
	int ind = l;
	int lPtr = 0;
    int revPair = 0;
    for(int i = 0; i < s2; i++) {
        while(lPtr < s1 && lVec[lPtr] <= 2ll * rVec[i]) {
            lPtr++;
        }
        revPair += s1 - lPtr;
    }

    while(lInd < s1 && rInd < s2) {
        if(lVec[lInd] <= rVec[rInd]) {
            nums[ind] = lVec[lInd];
            ind++; lInd++;
        } else {
            nums[ind] = rVec[rInd];
            ind++; rInd++;
        }
    }

    while(lInd < s1) {
        nums[ind] = lVec[lInd];
        ind++; lInd++;
    }

    while(rInd < s2) {
        nums[ind] = rVec[rInd];
        ind++; rInd++;
    }

    return revPair;
    }

void copy(vector<int> &vec1, vector<int> &vec2, int l, int r) {
	int s = r - l + 1;
	for(int i = 0; i < s; i++) {
		vec2[i] = vec1[i + l];
}
}
public:
    int reversePairs(vector<int>& nums) {
        return mergeSort(nums, 0, nums.size() - 1);
    }
};

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