edit 17

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_17.java

@@ -3,24 +3,26 @@
 import java.util.*;
 
 /**
+ * 17. Letter Combinations of a Phone Number
+ *
  * Given a digit string, return all possible letter combinations that the number could represent.
-
- A mapping of digit to letters (just like on the telephone buttons) is given below.
+ * A mapping of digit to letters (just like on the telephone buttons) is given below.
 
  Input:Digit string "23"
  Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
+
  Note:
  Although the above answer is in lexicographical order, your answer could be in any order you want.
  */
 
 public class _17 {
 
     public List<String> letterCombinations(String digits) {
-        String[] digits2Letters = new String[]{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
-
         List<String> result = new ArrayList();
         if(digits.length() == 0) return result;
 
+        String[] digits2Letters = new String[]{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
+
         result.add("");//this line is important, otherwise result is empty and Java will default it to an empty String
         for(int i = 0; i < digits.length(); i++){
             result = combine(digits2Letters[digits.charAt(i)-'0'], result);

src/test/java/com/fishercoder/_17Test.java

@@ -0,0 +1,41 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._17;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class _17Test {
+    private static _17 test;
+    private static String digits;
+    private static List<String> expected;
+    private static List<String> actual;
+
+    @BeforeClass
+    public static void setup(){
+        test = new _17();
+    }
+
+    @Test
+    public void test1(){
+        digits = "2";
+        actual = test.letterCombinations(digits);
+        expected = new ArrayList<>(Arrays.asList("a", "b", "c"));
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    public void test2(){
+        digits = "23";
+        actual = test.letterCombinations(digits);
+        expected = new ArrayList<>(Arrays.asList("ad","ae","af","bd","be","bf","cd","ce","cf"));
+        /**order doesn't matter, so we check like below*/
+        assertTrue(expected.containsAll(actual) && actual.containsAll(expected));
+    }
+}