Update 0242.有效的字母异位词.md

添加新的python题解方法

problems/0242.有效的字母异位词.md

@@ -154,6 +154,16 @@ class Solution:
 
         return s_dict == t_dict
 ```
+Python写法三(没有使用数组作为哈希表，只是介绍Counter这种更方便的解题思路)：
+
+```python
+class Solution(object):
+    def isAnagram(self, s: str, t: str) -> bool:
+        from collections import Counter
+        a_count = Counter(s)
+        b_count = Counter(t)
+        return a_count == b_count
+```
 
 Go：