Added task 17.

Author: javadev

src/main/java/s0017.letter.combinations.of.a.phone.number/Solution.java

@@ -0,0 +1,49 @@
+package s0017.letter.combinations.of.a.phone.number;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class Solution {
+    public List<String> letterCombinations(String digits) {
+        if (digits.isEmpty()) return Collections.emptyList();
+        List<String> words = new ArrayList<>();
+        char[] word = new char[digits.length()];
+        helper(digits, word, 0, words);
+        return words;
+    }
+
+    private void helper(String digits, char[] word, int cur, List<String> words) {
+        if (cur == digits.length()) {
+            words.add(new String(word));
+        } else {
+            for (char ch : charsForDigit(digits.charAt(cur))) {
+                word[cur] = ch;
+                helper(digits, word, cur + 1, words);
+            }
+        }
+    }
+
+    private char[] charsForDigit(char digit) {
+        switch (digit) {
+            case '2':
+                return new char[] {'a', 'b', 'c'};
+            case '3':
+                return new char[] {'d', 'e', 'f'};
+            case '4':
+                return new char[] {'g', 'h', 'i'};
+            case '5':
+                return new char[] {'j', 'k', 'l'};
+            case '6':
+                return new char[] {'m', 'n', 'o'};
+            case '7':
+                return new char[] {'p', 'q', 'r', 's'};
+            case '8':
+                return new char[] {'t', 'u', 'v'};
+            case '9':
+                return new char[] {'w', 'x', 'y', 'z'};
+            default:
+                return new char[] {};
+        }
+    }
+}

src/test/java/s0017.letter.combinations.of.a.phone.number/SolutionTest.java

@@ -0,0 +1,15 @@
+package s0017.letter.combinations.of.a.phone.number;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void letterCombinations() {
+        assertThat(
+                new Solution().letterCombinations("23").toString(),
+                equalTo("[ad, ae, af, bd, be, bf, cd, ce, cf]"));
+    }
+}