🟡 Port problem 17 (Letter Combinations of a Phone Number) from Swift to C++23

solutions/0001-0100/0017-letter-combinations-of-a-phone-number/README.md

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+# 17. Letter Combinations of a Phone Number
+
+[LeetCode problem 17](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)
+
+- **Difficulty**: Medium
+- **Tags**: hash-table, string, backtracking
+
+## Description
+
+Given a string of digits from `2` to `9`, return all possible letter
+combinations the digits could represent on a phone keypad, in any order.

solutions/0001-0100/0017-letter-combinations-of-a-phone-number/solution.cpp

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+#include "solution.hpp"
+
+#include <unordered_map>
+
+namespace leetcode::p0017 {
+
+std::vector<std::string> solve(const std::string& digits) {
+    if (digits.empty()) {
+        return {};
+    }
+
+    static const std::unordered_map<char, std::string> keypad{
+        {'2', "abc"},
+        {'3', "def"},
+        {'4', "ghi"},
+        {'5', "jkl"},
+        {'6', "mno"},
+        {'7', "pqrs"},
+        {'8', "tuv"},
+        {'9', "wxyz"},
+    };
+
+    std::vector<std::string> result{""};
+
+    for (char digit : digits) {
+        std::vector<std::string> next;
+        for (const auto& combination : result) {
+            for (char letter : keypad.at(digit)) {
+                next.push_back(combination + letter);
+            }
+        }
+        result = std::move(next);
+    }
+
+    return result;
+}
+
+} // namespace leetcode::p0017

solutions/0001-0100/0017-letter-combinations-of-a-phone-number/solution.hpp

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+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace leetcode::p0017 {
+
+// optimized: O(4^n * n) time (n = digits.size()), O(4^n * n) space — build
+// the combinations iteratively, one digit at a time.
+std::vector<std::string> solve(const std::string& digits);
+
+} // namespace leetcode::p0017

solutions/0001-0100/0017-letter-combinations-of-a-phone-number/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 17", "[p0017]") {
+    CHECK(leetcode::p0017::solve("").empty());
+    CHECK(leetcode::p0017::solve("2") == std::vector<std::string>{"a", "b", "c"});
+    CHECK(leetcode::p0017::solve("23") ==
+          std::vector<std::string>{"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"});
+}