feat: throw for wrong inputs

Author: vil02

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

@@ -41,8 +41,13 @@ private static List<String> generateCombinations(int[] numbers, int index, Strin
             return new ArrayList<>(Collections.singletonList(current.toString()));
         }
 
+        final var number = numbers[index];
+        if (number < 0 || number > 9) {
+            throw new IllegalArgumentException("Input numbers must in the range [0, 9]");
+        }
+
+        final String letters = KEYPAD.get(number); // Get corresponding letters for the current number
         List<String> combinations = new ArrayList<>();
-        final String letters = KEYPAD.get(numbers[index]); // Get corresponding letters for the current number
 
         // Iterate over each letter and recurse to generate further combinations
         for (char letter : letters.toCharArray()) {

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

@@ -1,6 +1,7 @@
 package com.thealgorithms.strings;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Arrays;
 import java.util.List;
@@ -17,10 +18,20 @@ public void testLetterCombinationsOfPhoneNumber(int[] numbers, List<String> expe
         assertEquals(expectedOutput, LetterCombinationsOfPhoneNumber.getCombinations(numbers));
     }
 
+    @ParameterizedTest
+    @MethodSource("wrongInputs")
+    void throwsForWrongInput(int[] numbers) {
+        assertThrows(IllegalArgumentException.class, () -> LetterCombinationsOfPhoneNumber.getCombinations(numbers));
+    }
+
     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[] {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")));
     }
+
+    private static Stream<Arguments> wrongInputs() {
+        return Stream.of(Arguments.of(new int[] {-1}), Arguments.of(new int[] {10}), Arguments.of(new int[] {2, 2, -1, 0}), Arguments.of(new int[] {0, 0, 0, 10}));
+    }
 }