17. 电话号码的字母组合

Author: w2xi

README.md

@@ -20,6 +20,7 @@
 |13|[罗马数字转整数](https://leetcode.cn/problems/roman-to-integer/)|[JavaScript](./algorithms/roman-to-integer.js)|Easy|
 |14|[最长公共前缀](https://leetcode-cn.com/problems/longest-common-prefix/)|[JavaScript](./algorithms/longest-common-prefix.js)|Easy|
 |15|[三数之和](https://leetcode.cn/problems/3sum/)|[JavaScript](./algorithms/3sum.js)|Medium|
+|17|[电话号码的字母组合](https://leetcode.cn/problems/letter-combinations-of-a-phone-number/)|[JavaScript](./algorithms/letter-combinations-of-a-phone-number.js)|Medium|
 |18|[四数之和](https://leetcode.cn/problems/4sum/)|[JavaScript](./algorithms/4sum.js)|Medium|
 |19|[删除链表的倒数第 N 个结点](https://leetcode.cn/problems/remove-nth-node-from-end-of-list/)|[JavaScript](./algorithms/remove-nth-node-from-end-of-list.js)|Medium|
 |20|[有效的括号](https://leetcode.cn/problems/valid-parentheses/)|[JavaScript](./algorithms/valid-parentheses.js)|Easy|

algorithms/letter-combinations-of-a-phone-number.js

@@ -0,0 +1,39 @@
+/**
+ * @param {string} digits
+ * @return {string[]}
+ */
+var letterCombinations = function (digits) {
+  if (!digits.length) return [];
+  const result = [];
+  const map = [
+    "", // 0
+    "", // 1
+    "abc", // 2
+    "def", // 3
+    "ghi", // 4
+    "jkl", // 5
+    "mno", // 6
+    "pqrs", // 7
+    "tuv", // 8
+    "wxyz", // 9
+  ];
+  let str = "";
+
+  const backtracking = (digits, startIndex = 0) => {
+    if (str.length === digits.length) {
+      result.push(str);
+      return;
+    }
+    for (let i = startIndex; i < digits.length; i++) {
+      const chars = map[digits[i]];
+      for (let j = 0; j < chars.length; j++) {
+        str += chars[j];
+        backtracking(digits, i + 1);
+        str = str.slice(0, str.length - 1);
+      }
+    }
+  };
+  backtracking(digits);
+
+  return result;
+};