Merge pull request #394 from borninfreedom/master

update有效的字母异位词，python代码添加了更pythonic的写法

problems/0242.有效的字母异位词.md

@@ -130,7 +130,27 @@ class Solution:
         return True
 ```
 
+Python写法二（没有使用数组作为哈希表，只是介绍defaultdict这样一种解题思路）：
+
+```python
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        from collections import defaultdict
+        
+        s_dict = defaultdict(int)
+        t_dict = defaultdict(int)
+
+        for x in s:
+            s_dict[x] += 1
+        
+        for x in t:
+            t_dict[x] += 1
+
+        return s_dict == t_dict
+```
+
 Go：
+
 ```go
 func isAnagram(s string, t string) bool {
     if len(s)!=len(t){