🟢 Port problem 1512 (Number of Good Pairs) from Python to C++23

solutions/1501-1600/1512-number-of-good-pairs/README.md

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+# 1512. Number of Good Pairs
+
+[LeetCode problem 1512](https://leetcode.com/problems/number-of-good-pairs/)
+
+- **Difficulty**: Easy
+- **Tags**: array, hash-table, math, counting
+
+## Description
+
+Given an array of integers `nums`, return the number of good pairs. A pair
+`(i, j)` is good if `nums[i] == nums[j]` and `i < j`.

solutions/1501-1600/1512-number-of-good-pairs/solution.cpp

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+#include "solution.hpp"
+
+#include <unordered_map>
+
+namespace leetcode::p1512 {
+
+int solve(const std::vector<int>& nums) {
+    std::unordered_map<int, int> counts;
+    int result = 0;
+
+    for (int value : nums) {
+        auto& count = counts[value];
+        result += count;
+        ++count;
+    }
+
+    return result;
+}
+
+int solveBruteForce(const std::vector<int>& nums) {
+    int result = 0;
+
+    for (std::size_t i = 0; i < nums.size(); ++i) {
+        for (std::size_t j = i + 1; j < nums.size(); ++j) {
+            if (nums[i] == nums[j]) {
+                ++result;
+            }
+        }
+    }
+
+    return result;
+}
+
+} // namespace leetcode::p1512

solutions/1501-1600/1512-number-of-good-pairs/solution.hpp

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+#pragma once
+
+#include <vector>
+
+namespace leetcode::p1512 {
+
+// optimized: O(n) time, O(n) space — count occurrences per value, then sum
+// count * (count - 1) / 2 for each one.
+int solve(const std::vector<int>& nums);
+
+// brute-force: O(n^2) time, O(1) space — check every pair.
+int solveBruteForce(const std::vector<int>& nums);
+
+} // namespace leetcode::p1512

solutions/1501-1600/1512-number-of-good-pairs/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 1512 - optimized", "[p1512]") {
+    CHECK(leetcode::p1512::solve({1, 2, 3, 1, 1, 3}) == 4);
+    CHECK(leetcode::p1512::solve({1, 1, 1, 1}) == 6);
+    CHECK(leetcode::p1512::solve({1, 2, 3}) == 0);
+}
+
+TEST_CASE("Problem 1512 - brute force", "[p1512]") {
+    CHECK(leetcode::p1512::solveBruteForce({1, 2, 3, 1, 1, 3}) == 4);
+    CHECK(leetcode::p1512::solveBruteForce({1, 1, 1, 1}) == 6);
+    CHECK(leetcode::p1512::solveBruteForce({1, 2, 3}) == 0);
+}