<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 solve(vector<int>& nums, int l, int r) {
        int m = l + (r - l) / 2, lp = 0, rp = 0, idx = l, lp2 = 0, cnt;
        if (l >= r)
            return;
        solve(nums, l, m);
        solve(nums, m + 1, r);
        vector<int64_t> left(nums.begin() + l, nums.begin() + m + 1);
        vector<int64_t> right(nums.begin() + m + 1, nums.begin() + r + 1);
        while (lp < left.size() || rp < right.size()) {
            if (lp == left.size()) {
                ans += left.end() - upper_bound(left.begin(), left.end(), right[rp] << 1);
                nums[idx++] = right[rp++];
            }
            else if (rp == right.size())
                nums[idx++] = left[lp++];
            else if (left[lp] < right[rp])
                nums[idx++] = left[lp++];
            else {
                ans += left.end() - upper_bound(left.begin(), left.end(), right[rp] << 1);
                nums[idx++] = right[rp++];
            }
        }

    }

    int reversePairs(vector<int>& nums) {
        solve(nums, 0, nums.size() - 1);
        return ans;
    }
};
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