Problem - Reverse Pairs

Given an integer array nums, count the number of reverse pairs: indices $(i, j)$ where $i < j$ and $nums[i] > 2 \cdot nums[j]$. Example: nums $= [1, 3, 2, 3, 1]$ returns $2$.

Reverse pairs: $(1, 4)$ since $3 > 2 \cdot 1$, and $(3, 4)$ since $3 > 2 \cdot 1$.

This is similar to inversions, but the condition is $a[i] > 2 \cdot a[j]$ instead of $a[i] > a[j]$. Approach: process right to left, query for elements less than $a[i] / 2$. Constraints: up to $5 \cdot 10^4$ elements, values up to $2^{31} - 1$.