add tests

Author: Samuel Facchinello

src/main/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumber.java

@@ -8,7 +8,7 @@
 public final class LetterCombinationsOfPhoneNumber {
 
     // Mapping of numbers to corresponding letters on a phone keypad
-    private static final Map<Integer, String> KEYPAD = Map.of(0, "", 1, "", 2, "abc", 3, "def", 4, "ghi", 5, "jkl", 6, "mno", 7, "pqrs", 8, "tuv", 9, "wxyz");
+    private static final Map<Integer, String> KEYPAD = Map.of(0, " ", 1, "", 2, "abc", 3, "def", 4, "ghi", 5, "jkl", 6, "mno", 7, "pqrs", 8, "tuv", 9, "wxyz");
 
     private LetterCombinationsOfPhoneNumber() {
     }

src/test/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumberTest.java

@@ -19,6 +19,8 @@ public void testLetterCombinationsOfPhoneNumber(int[] numbers, List<String> expe
 
     private static Stream<Arguments> provideTestCases() {
         return Stream.of(Arguments.of(null, List.of("")), Arguments.of(new int[] {}, List.of("")), Arguments.of(new int[] {2}, Arrays.asList("a", "b", "c")), Arguments.of(new int[] {2, 3}, Arrays.asList("ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf")),
-            Arguments.of(new int[] {2, 3, 4}, Arrays.asList("adg", "adh", "adi", "aeg", "aeh", "aei", "afg", "afh", "afi", "bdg", "bdh", "bdi", "beg", "beh", "bei", "bfg", "bfh", "bfi", "cdg", "cdh", "cdi", "ceg", "ceh", "cei", "cfg", "cfh", "cfi")));
+            Arguments.of(new int[] {2, 3, 4}, Arrays.asList("adg", "adh", "adi", "aeg", "aeh", "aei", "afg", "afh", "afi", "bdg", "bdh", "bdi", "beg", "beh", "bei", "bfg", "bfh", "bfi", "cdg", "cdh", "cdi", "ceg", "ceh", "cei", "cfg", "cfh", "cfi")),
+            Arguments.of(new int[] {3, 3}, Arrays.asList("dd", "de", "df", "ed", "ee", "ef", "fd", "fe", "ff")), Arguments.of(new int[] {8, 4}, Arrays.asList("tg", "th", "ti", "ug", "uh", "ui", "vg", "vh", "vi")), Arguments.of(new int[] {2, 0}, Arrays.asList("a ", "b ", "c ")),
+            Arguments.of(new int[] {9, 2}, Arrays.asList("wa", "wb", "wc", "xa", "xb", "xc", "ya", "yb", "yc", "za", "zb", "zc")));
     }
 }