feat: add Letter Combinations of a Phone Number

Author: dukeluo

README.md

@@ -78,6 +78,7 @@ Roadmap: https://neetcode.io/roadmap
 | 40   | [Combination Sum II](https://leetcode.com/problems/combination-sum-ii/description/)                                                                               | Medium     | [ts](./TypeScript/40.combination-sum-ii.ts)                                                   | Backtracking     |
 | 79   | [Word Search](https://leetcode.com/problems/word-search/)                                                                                                         | Medium     | [ts](./TypeScript/79.word-search.ts)                                                          | Backtracking     |
 | 131  | [Palindrome Partitioning](https://leetcode.com/problems/palindrome-partitioning/description/)                                                                     | Medium     | [ts](./TypeScript/131.palindrome-partitioning.ts)                                             | Backtracking     |
+| 17   | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/)                                         | Medium     | [ts](./TypeScript/17.letter-combinations-of-a-phone-number.ts)                                | Backtracking     |
 
 ### Others
 

TypeScript/17.letter-combinations-of-a-phone-number.ts

@@ -0,0 +1,39 @@
+function letterCombinations(digits: string): string[] {
+  const map = {
+    "2": ["a", "b", "c"],
+    "3": ["d", "e", "f"],
+    "4": ["g", "h", "i"],
+    "5": ["j", "k", "l"],
+    "6": ["m", "n", "o"],
+    "7": ["p", "q", "r", "s"],
+    "8": ["t", "u", "v"],
+    "9": ["w", "x", "y", "z"],
+  };
+  const combinations: string[][] = [];
+  const combination: string[] = [];
+
+  function backtrace(index) {
+    if (index > digits.length) {
+      return;
+    }
+    if (index === digits.length) {
+      combinations.push(combination.slice());
+      return;
+    }
+    for (let i = 0; i < map[digits.charAt(index)].length; i++) {
+      combination.push(map[digits.charAt(index)][i]);
+      backtrace(index + 1);
+      combination.pop();
+    }
+  }
+
+  backtrace(0);
+
+  if (!digits) {
+    return [];
+  }
+
+  return combinations.map((c) => c.join(""));
+}
+
+console.log(letterCombinations("23"));