solved: Letter combinations of a phone number

Author: lambiengcode

README.md

@@ -25,6 +25,7 @@
   - [Text Justification](#text-justification)
   - [3 Sum](#3-sum)
   - [3 Sum Closest](#3-sum-closest)
+  - [Letter combinations of a phone number](#letter-combinations-of-a-phone-number)
 
 ### String to Int - Atoi
 
@@ -676,4 +677,46 @@ int threeSumClosest(List<int> nums, int target) {
 
   return result;
 }
+```
+
+### Letter combinations of a phone number
+
+```dart
+const Map<int, List<String>> chars = {
+  2: ['a', 'b', 'c'],
+  3: ['d', 'e', 'f'],
+  4: ['g', 'h', 'i'],
+  5: ['k', 'j', 'l'],
+  6: ['m', 'n', 'o'],
+  7: ['p', 'q', 'r', 's'],
+  8: ['t', 'u', 'v'],
+  9: ['w', 'x', 'y', 'z'],
+};
+
+List<String> letterCombinations(String digits) {
+  if (digits.isEmpty) return [];
+
+  final List<String> result = [];
+
+  lookupChar(0, "", digits.length, digits, result);
+
+  return result;
+}
+
+void lookupChar(
+  int index,
+  String res,
+  int length,
+  String digits,
+  List<String> result,
+) {
+  if (index == digits.length)
+    result.add(res);
+  else {
+    final List<String> letters = chars[int.parse(digits[index])]!;
+    for (int i = 0; i < letters.length; i++) {
+      lookupChar(index + 1, res + letters[i], length, digits, result);
+    }
+  }
+}
 ```

src/letter_combinations_of_a_phone_number.dart

@@ -0,0 +1,45 @@
+void main(List<String> args) {
+  List<String> cases = ["23", "", "2", "9"];
+
+  for (final testcase in cases) {
+    print(letterCombinations(testcase));
+  }
+}
+
+const Map<int, List<String>> chars = {
+  2: ['a', 'b', 'c'],
+  3: ['d', 'e', 'f'],
+  4: ['g', 'h', 'i'],
+  5: ['k', 'j', 'l'],
+  6: ['m', 'n', 'o'],
+  7: ['p', 'q', 'r', 's'],
+  8: ['t', 'u', 'v'],
+  9: ['w', 'x', 'y', 'z'],
+};
+
+List<String> letterCombinations(String digits) {
+  if (digits.isEmpty) return [];
+
+  final List<String> result = [];
+
+  lookupChar(0, "", digits.length, digits, result);
+
+  return result;
+}
+
+void lookupChar(
+  int index,
+  String res,
+  int length,
+  String digits,
+  List<String> result,
+) {
+  if (index == digits.length)
+    result.add(res);
+  else {
+    final List<String> letters = chars[int.parse(digits[index])]!;
+    for (int i = 0; i < letters.length; i++) {
+      lookupChar(index + 1, res + letters[i], length, digits, result);
+    }
+  }
+}