Number of Pairs of Interchangeable Rectangles — C++

// Time:  O(n)
// Space: O(n)

class Solution {
public:
    long long interchangeableRectangles(vector<vector<int>>& rectangles) {
        unordered_map<int, unordered_map<int, int>> lookup;
        for (const auto& rect : rectangles) {
            const int g = gcd(rect[0], rect[1]);  // Time: O(logx) ~= O(1)
            ++lookup[rect[0] / g][rect[1] / g];
        }
        int64_t result = 0;
        for (const auto& [_, kvp] : lookup) {
            for (const auto& [_, c] : kvp) {
                result += static_cast<int64_t>(c) * (c - 1) / 2;
            }
        }
        return result;
    }
};