diff --git a/cpp/2205/220509.cpp b/cpp/2205/220509.cpp
new file mode 100644
index 0000000..3665730
--- /dev/null
+++ b/cpp/2205/220509.cpp
@@ -0,0 +1,61 @@
+#include <vector>
+#include <string>
+#include <functional>
+#include <iostream>
+
+/**
+ * 17. Letter Combinations of a Phone Number
+ * Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
+ * A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+ */
+
+class Solution {
+private:
+	const inline static char* key[] {
+		"abc",
+		"def",
+		"ghi",
+		"jkl",
+		"mno",
+		"pqrs",
+		"tuv",
+		"wxyz"
+	};
+
+public:
+	static std::vector<std::string> letterCombinations(const std::string& digits) {
+		if (digits.empty())
+			return {};
+
+		std::vector<std::string> ret;
+		ret.reserve(1 << (digits.length() + 2));
+		std::string str;
+		str.reserve(8);
+		const int n = digits.length();
+
+		std::function<void(int)> dfs = [&](int depth) {
+			if (depth >= n) {
+				ret.push_back(str);
+				return;
+			}
+
+			for (const char* ptr = key[digits[depth] - '2']; *ptr; ++ptr) {
+				str.push_back(*ptr);
+				dfs(depth + 1);
+				str.pop_back();
+			}
+		};
+
+		dfs(0);
+
+		return ret;
+	}
+};
+
+int main() {
+	auto r = Solution::letterCombinations("2");
+	for (const auto& i : r) {
+		std::cout << i << std::endl;
+	}
+	return 0;
+}
diff --git a/cpp/2205/CMakeLists.txt b/cpp/2205/CMakeLists.txt
index 293aad9..364e3a4 100644
--- a/cpp/2205/CMakeLists.txt
+++ b/cpp/2205/CMakeLists.txt
@@ -3,4 +3,4 @@ PROJECT(2205)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2205 220509-CN.cpp)
+ADD_EXECUTABLE(2205 220509.cpp)