add python solution to 0541.reverseStrII

problems/0541.反转字符串II.md

@@ -134,6 +134,36 @@ class Solution {
 ```
 
 Python：
+```python3
+
+class Solution(object):
+    def reverseStr(self, s, k):
+        """
+        :type s: str
+        :type k: int
+        :rtype: str
+        """
+        from functools import reduce
+        # turn s into a list 
+        s = list(s)
+        
+        # another way to simply use a[::-1], but i feel this is easier to understand
+        def reverse(s):
+            left, right = 0, len(s) - 1
+            while left < right:
+                s[left], s[right] = s[right], s[left]
+                left += 1
+                right -= 1
+            return s
+        
+        # make sure we reverse each 2k elements 
+        for i in range(0, len(s), 2*k):
+            s[i:(i+k)] = reverse(s[i:(i+k)])
+        
+        # combine list into str.
+        return reduce(lambda a, b: a+b, s)
+        
+```
 
 
 Go：