🟢 Port problem 387 (First Unique Character in a String) from Swift to C++23

solutions/0301-0400/0387-first-unique-character-in-a-string/README.md

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+# 387. First Unique Character in a String
+
+[LeetCode problem 387](https://leetcode.com/problems/first-unique-character-in-a-string/)
+
+- **Difficulty**: Easy
+- **Tags**: hash-table, string, queue, counting
+
+## Description
+
+Given a string `s`, return the index of the first non-repeating character,
+or `-1` if none exists.

solutions/0301-0400/0387-first-unique-character-in-a-string/solution.cpp

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+#include "solution.hpp"
+
+#include <array>
+
+namespace leetcode::p0387 {
+
+int solve(const std::string& s) {
+    std::array<int, 26> counts{};
+
+    for (char c : s) {
+        ++counts.at(static_cast<std::size_t>(c - 'a'));
+    }
+
+    for (std::size_t i = 0; i < s.size(); ++i) {
+        if (counts.at(static_cast<std::size_t>(s[i] - 'a')) == 1) {
+            return static_cast<int>(i);
+        }
+    }
+
+    return -1;
+}
+
+} // namespace leetcode::p0387

solutions/0301-0400/0387-first-unique-character-in-a-string/solution.hpp

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+#pragma once
+
+#include <string>
+
+namespace leetcode::p0387 {
+
+// optimized: O(n) time, O(1) space — fixed 26-slot count array (assumes
+// lowercase English letters, per the problem's constraints), two passes.
+int solve(const std::string& s);
+
+} // namespace leetcode::p0387

solutions/0301-0400/0387-first-unique-character-in-a-string/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 387", "[p0387]") {
+    CHECK(leetcode::p0387::solve("leetcode") == 0);
+    CHECK(leetcode::p0387::solve("loveleetcode") == 2);
+    CHECK(leetcode::p0387::solve("aabb") == -1);
+}