cpp: add «17. Letter Combinations of a Phone Number»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cpp/letter-combinations-of-a-phone-number.cpp

@@ -0,0 +1,57 @@
+#include <array>
+#include <cassert>
+#include <string>
+#include <vector>
+
+struct Solution {
+    std::vector<std::string> letterCombinations(const std::string &digits) {
+        solver s(digits);
+        s.solve();
+        return s.combinations;
+    }
+
+  private:
+    struct solver {
+        const std::string &digits;
+        std::vector<std::string> combinations;
+
+        solver(const std::string &digits) : digits(digits) {}
+        void solve() {
+            std::string s;
+            solve(s, 0);
+        }
+
+      private:
+        std::array<std::string, 8> LETTERS{"abc", "def",  "ghi", "jkl",
+                                           "mno", "pqrs", "tuv", "wxyz"};
+
+        void solve(std::string &s, int i) {
+            if (i < 0 || i >= static_cast<int>(digits.size())) {
+                if (!s.empty()) {
+                    combinations.emplace_back(s);
+                }
+
+                return;
+            }
+
+            for (char c : LETTERS[digits[i] - '2']) {
+                s.push_back(c);
+                solve(s, i + 1);
+                s.pop_back();
+            }
+        }
+    };
+};
+
+int main() {
+    Solution s;
+
+    assert((s.letterCombinations("23") ==
+            std::vector<std::string>{"ad", "ae", "af", "bd", "be", "bf", "cd",
+                                     "ce", "cf"}));
+    assert((s.letterCombinations("") == std::vector<std::string>{}));
+    assert(
+        (s.letterCombinations("2") == std::vector<std::string>{"a", "b", "c"}));
+
+    return 0;
+}