Merge pull request youngyangyang04#1179 from xiaofei-2020/back05

添加（0017.电话号码的字母组合.md）：增加typesript版本

Author: youngyangyang04

problems/0017.电话号码的字母组合.md

@@ -420,6 +420,40 @@ var letterCombinations = function(digits) {
 };
 ```
 
+## TypeScript
+
+```typescript
+function letterCombinations(digits: string): string[] {
+    if (digits === '') return [];
+    const strMap: { [index: string]: string[] } = {
+        1: [],
+        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 resArr: string[] = [];
+    function backTracking(digits: string, curIndex: number, route: string[]): void {
+        if (curIndex === digits.length) {
+            resArr.push(route.join(''));
+            return;
+        }
+        let tempArr: string[] = strMap[digits[curIndex]];
+        for (let i = 0, length = tempArr.length; i < length; i++) {
+            route.push(tempArr[i]);
+            backTracking(digits, curIndex + 1, route);
+            route.pop();
+        }
+    }
+    backTracking(digits, 0, []);
+    return resArr;
+};
+```
+
 ## C
 
 ```c