添加（0242.有效的字母异位词.md）：增加typescript版本

problems/0242.有效的字母异位词.md

@@ -214,7 +214,23 @@ var isAnagram = function(s, t) {
 };
 ```
 
+TypeScript：
+
+```typescript
+function isAnagram(s: string, t: string): boolean {
+    if (s.length !== t.length) return false;
+    let helperArr: number[] = new Array(26).fill(0);
+    let pivot: number = 'a'.charCodeAt(0);
+    for (let i = 0, length = s.length; i < length; i++) {
+        helperArr[s.charCodeAt(i) - pivot]++;
+        helperArr[t.charCodeAt(i) - pivot]--;
+    }
+    return helperArr.every(i => i === 0);
+};
+```
+
 Swift：
+
 ```Swift
 func isAnagram(_ s: String, _ t: String) -> Bool {
     if s.count != t.count {