add js solution for leetcode 17

Author: swiftwind0405

README.md

@@ -397,7 +397,7 @@ LeetCode
 |20|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| [js](./algorithms/validParentheses/validParentheses.js) |Easy|
 |19|[Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/)| [js](./algorithms/removeNthNodeFromEndOfList/removeNthNodeFromEndOfList.js) |Easy|
 |18|[4Sum](https://leetcode.com/problems/4sum/)| [js](./algorithms/4Sum/4Sum.js) |Medium|
-|17|[Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)| |Medium|
+|17|[Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)| [js](./algorithms/letterCombinationsOfAPhoneNumber/letterCombinationsOfAPhoneNumber.js) |Medium|
 |16|[3Sum Closest](https://leetcode.com/problems/3sum-closest/)| |Medium|
 |15|[3Sum](https://leetcode.com/problems/3sum/)| [js](./algorithms/3Sum/3Sum.js) |Medium|
 |14|[Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/)| [js](./algorithms/longestCommonPrefix/longestCommonPrefix.js) |Easy|

algorithms/letterCombinationsOfAPhoneNumber/letterCombinationsOfAPhoneNumber.js

@@ -0,0 +1,33 @@
+var letterCombinations = function (digits) {
+  if (digits.length == 0) return [];
+
+  const map = new Map(),
+    res = [];
+  // 初始化字典映射
+  map.set("2", "abc");
+  map.set("3", "def");
+  map.set("4", "ghi");
+  map.set("5", "jkl");
+  map.set("6", "mno");
+  map.set("7", "pqrs");
+  map.set("8", "tuv");
+  map.set("9", "wxyz");
+
+  const dfs = (curStr, i) => {
+    // 指针越界，递归的出口
+    if (i > digits.length - 1) {
+      // 将解推入res
+      res.push(curStr);
+      // 结束当前递归分支，进入另一个递归分支
+      return;
+    }
+
+    const letters = map.get(digits[i]);
+    // 不同的字母选择代表不同的递归分支
+    for (const l of letters) {
+      dfs(curStr + l, i + 1);
+    }
+  };
+  dfs("", 0);
+  return res;
+};