Add test case for LetterCombinationsOfPhoneNumber (#3662)

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

@@ -0,0 +1,50 @@
+package com.thealgorithms.strings;
+
+import java.util.*;
+
+public class LetterCombinationsOfPhoneNumber {
+
+    static Character[][] numberToCharMap;
+
+    protected static List<String> printWords(int[] numbers, int len, int numIndex, String s) {
+        if (len == numIndex) {
+            return new ArrayList<>(Collections.singleton(s));
+        }
+
+        List<String> stringList = new ArrayList<>();
+
+        for (int i = 0; i < numberToCharMap[numbers[numIndex]].length; i++) {
+            String sCopy = String.copyValueOf(s.toCharArray());
+            sCopy =
+                sCopy.concat(numberToCharMap[numbers[numIndex]][i].toString());
+            stringList.addAll(printWords(numbers, len, numIndex + 1, sCopy));
+        }
+        return stringList;
+    }
+
+    private static void printWords(int[] numbers) {
+        generateNumberToCharMap();
+        List<String> stringList = printWords(numbers, numbers.length, 0, "");
+        stringList.stream().forEach(System.out::println);
+    }
+
+    protected static void generateNumberToCharMap() {
+        numberToCharMap = new Character[10][5];
+        numberToCharMap[0] = new Character[] { '\0' };
+        numberToCharMap[1] = new Character[] { '\0' };
+        numberToCharMap[2] = new Character[] { 'a', 'b', 'c' };
+        numberToCharMap[3] = new Character[] { 'd', 'e', 'f' };
+        numberToCharMap[4] = new Character[] { 'g', 'h', 'i' };
+        numberToCharMap[5] = new Character[] { 'j', 'k', 'l' };
+        numberToCharMap[6] = new Character[] { 'm', 'n', 'o' };
+        numberToCharMap[7] = new Character[] { 'p', 'q', 'r', 's' };
+        numberToCharMap[8] = new Character[] { 't', 'u', 'v' };
+        numberToCharMap[9] = new Character[] { 'w', 'x', 'y', 'z' };
+    }
+
+    // Driver code
+    public static void main(String[] args) {
+        int number[] = { 2, 3, 4 };
+        printWords(number);
+    }
+}