Letter Combinations of a Phone Number

Recursion/LetterCombination.java

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+import java.util.*;
+
+public class LetterCombination {
+
+    // complexity is o(3 ^n . n) and space complexity is o(n) + o(n) + o(3 ^n *n) : n is the len of digit
+
+    private static Map<Character, String> letters = Map.of(
+            '2', "abc", '3', "def", '4', "ghi", '5', "jkl",
+            '6', "mno", '7', "pqrs", '8', "tuv", '9', "wxyz");
+
+
+    public static List<String> letterCombinations(String digits) {
+        int len = digits.length();
+        if (len == 0) return (new ArrayList<>()) ;
+        List<String> result = new ArrayList<>();
+        List<Character> slate = new ArrayList<>();
+        letterCombinationsHelper(digits, 0, slate, result);
+        return result;
+    }
+
+    private static void letterCombinationsHelper(String digits, int digitsIdx, List<Character> slate, List<String> result) {
+        if (digits.length() == digitsIdx) {
+            StringBuilder sb = new StringBuilder();
+            for (Character ch : slate)
+                sb.append(ch);
+            result.add(sb.toString());
+            return;
+        }
+
+        if (digits.charAt(digitsIdx) == 1) {
+            letterCombinationsHelper(digits, digitsIdx + 1, slate, result);
+        }
+
+        String let = letters.get(digits.charAt(digitsIdx));
+        for (Character l : let.toCharArray()) {
+            slate.add(l);
+            letterCombinationsHelper(digits, digitsIdx + 1, slate, result);
+            slate.remove(slate.size() - 1);
+        }
+    }
+
+    public static void main(String[] args) {
+        List<String> l = letterCombinations("23");
+        System.out.println(l);
+
+    }
+}
+
+
+
+