Add solution #17

Author: JoshCrozier

README.md

@@ -14,6 +14,7 @@
 8|[String to Integer (atoi)](./0008-string-to-integer-atoi.js)|Medium|
 10|[Regular Expression Matching](./0010-regular-expression-matching.js)|Hard|
 14|[Longest Common Prefix](./0014-longest-common-prefix.js)|Easy|
+17|[Letter Combinations of a Phone Number](./0017-letter-combinations-of-a-phone-number.js)|Medium|
 27|[Remove Element](./0027-remove-element.js)|Easy|
 31|[Next Permutation](./0031-next-permutation.js)|Medium|
 36|[Valid Sudoku](./0036-valid-sudoku.js)|Medium|

solutions/0017-letter-combinations-of-a-phone-number.js

@@ -0,0 +1,47 @@
+/**
+ * 17. Letter Combinations of a Phone Number
+ * https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+ * Difficulty: Medium
+ *
+ * Given a string containing digits from 2-9 inclusive, return all possible
+ * letter combinations that the number could represent. Return the answer
+ * in any order.
+ *
+ * A mapping of digit to letters (just like on the telephone buttons) is
+ * given below. Note that 1 does not map to any letters.
+ */
+
+/**
+ * @param {string} digits
+ * @return {string[]}
+ */
+var letterCombinations = function(digits) {
+  if (!digits || !digits.length) return [];
+
+  const map = {
+    2: 'abc',
+    3: 'def',
+    4: 'ghi',
+    5: 'jkl',
+    6: 'mno',
+    7: 'pqrs',
+    8: 'tuv',
+    9: 'wxyz'
+  };
+
+  if (digits.length === 1) {
+    return map[digits].split('');
+  }
+
+  const result = [];
+  const group1 = letterCombinations(digits.substr(0, 1));
+  const group2 = letterCombinations(digits.substr(1));
+
+  for (let i = 0; i < group1.length; i++) {
+    for (let j = 0; j < group2.length; j++) {
+      result.push(group1[i] + group2[j]);
+    }
+  }
+
+  return result;
+};