solves letter combinations of phone numbers

Author: anishLearnsToCode

README.md

@@ -25,6 +25,7 @@
 |  14  | [Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/)                                                                             |                               [![Java](assets/java.png)](src/LongestCommonPrefix.java) [![Python](assets/python.png)](python/longest_common_prefix.py)                               |                                                                                                                                   |
 |  15  | [3Sum](https://leetcode.com/problems/3sum)                                                                                                                |                                                                    [![Java](assets/java.png)](src/ThreeSum.java)                                                                     |                                                                                                                                   |
 |  16  | [3Sum Closest](https://leetcode.com/problems/3sum-closest)                                                                                                |                                                                 [![Java](assets/java.png)](src/ThreeSumClosest.java)                                                                 |                                                                                                                                   |
+|  17  | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number)                                              |                                                        [![Java](assets/java.png)](src/LetterCombinationsOfAPhoneNumber.java)                                                         |                                                                                                                                   |
 |  20  | [ValidParentheses](https://leetcode.com/problems/valid-parentheses/)                                                                                      |                                  [![Java](assets/java.png)](src/ValidParentheses.java) [![Python](assets/python.png)](python/valid_parentheses.py)                                   |                                                                                                                                   |
 |  21  | [Merge 2 Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)                                                                             |                                [![Java](assets/java.png)](src/Merge2SortedLists.java) [![Python](assets/python.png)](python/merge_2_sorted_lists.py)                                 |                                                                                                                                   |
 |  26  | [Remove Duplicates From Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)                                                 |                  [![Java](assets/java.png)](src/RemoveDuplicatesFromSortedArray.java) [![Python](assets/python.png)](python/remove_duplicates_from_sorted_array.py)                  |                                                                                                                                   |

src/LetterCombinationsOfAPhoneNumber.java

@@ -0,0 +1,35 @@
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+public class LetterCombinationsOfAPhoneNumber {
+    private static final Map<Character, Set<Character>> NUMBER_TO_CHARS = Map.ofEntries(
+            Map.entry('2', Set.of('a', 'b', 'c')),
+            Map.entry('3', Set.of('d', 'e', 'f')),
+            Map.entry('4', Set.of('g', 'h', 'i')),
+            Map.entry('5', Set.of('j', 'k', 'l')),
+            Map.entry('6', Set.of('m', 'n', 'o')),
+            Map.entry('7', Set.of('p', 'q', 'r', 's')),
+            Map.entry('8', Set.of('t', 'u', 'v')),
+            Map.entry('9', Set.of('w', 'x', 'y', 'z'))
+    );
+
+    public List<String> letterCombinations(String digits) {
+        if (digits.isEmpty()) return Collections.emptyList();
+        final List<String> result = new ArrayList<>();
+        addLetterCombinationsToResult(digits, 0, "", result);
+        return result;
+    }
+
+    private void addLetterCombinationsToResult(String s, int i, String current, List<String> result) {
+        if (i == s.length()) {
+            result.add(current);
+            return;
+        }
+        for (char character : NUMBER_TO_CHARS.get(s.charAt(i))) {
+            addLetterCombinationsToResult(s, i + 1, current + character, result);
+        }
+    }
+}