Leetcode 2001. Number of Pairs of Interchangeable Rectangles

Count how many index pairs of rectangles share the same width:height ratio. Normalize each (w,h) to its reduced form (e.g., divide by gcd) and use a hash map to tally frequencies, then sum frequency choose 2 (n up to 1e5 so avoid O(n^2) or floating-point comparisons).