<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:
    def reversePairs(self, nums: List[int]) -> int:
        def compute_pairs(a, b):
            i, j = 0, 0
            # a, b are sorted
            res = 0
            while i < len(a) and j < len(b):
                print(a[i]/2)
                # move j while nums[i] > 2 * nums[j]
                while j < len(b) and a[i]/2 > b[j]:
                    j += 1

                res += j
                # no more, so increment i
                i += 1

            # there might be another number in i that needs to be processed and paired
            while i < len(a):
                res += j
                i += 1

            return res

        def merge(l, r):
            if l == r:
                return ([nums[l]], 0)

            m = (l + r)//2
            left, l_res = merge(l, m)
            right, r_res = merge(m + 1, r)

            res = compute_pairs(left, right)

            new_array = [0] * (len(left) + len(right))

            i, j, x = 0, 0, 0
            while i < len(left) and j < len(right):
                if left[i] <= right[j]:
                    new_array[x] = left[i]
                    i += 1
                else:
                    new_array[x] = right[j]
                    j += 1
                x += 1

            # might have one more
            while i < len(left):
                new_array[x] = left[i]
                x += 1
                i += 1

            while j < len(right):
                new_array[x] = right[j]
                x += 1
                j += 1

            return new_array, l_res + r_res + res

        return merge(0, len(nums) - 1)[1]

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