Update README.md

README.md

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 LeetCode
 ======== 
 
-Up to date (2015-09-10), there are `263` Algorithms / `13` Database / `4` Shell questions on [LeetCode Online Judge](https://leetcode.com/).
+Up to date (2015-09-14), there are `264` Algorithms / `13` Database / `4` Shell questions on [LeetCode Online Judge](https://leetcode.com/).
 The number of questions is increasing recently.
-Here is the classification of all `280` questions.
+Here is the classification of all `281` questions.
 For more questions and solutions, you can see my [LintCode](https://github.com/kamyu104/LintCode) repository.
 I'll keep updating for full summary and better solutions. Stay tuned for updates.
 (Notes: "📖" means you need to subscribe to [LeetCode premium membership](https://leetcode.com/subscribe/) for the access to premium questions. )
@@ -168,6 +168,7 @@ Shell
   #  | Problem         |  Solution       |  Time           | Space           | Difficulty    | Tag          | Note
 -----|---------------- | --------------- | --------------- | --------------- | ------------- |--------------| -----
 239| [Sliding Window Maximum](https://leetcode.com/problems/sliding-window-maximum/)| [C++](./C++/sliding-window-maximum.cpp) [Python](./Python/sliding-window-maximum.py) | _O(n)_        | _O(k)_          | Hard           | EPI, LintCode |
+281| [Zigzag Iterator](https://leetcode.com/problems/sliding-window-maximum/)| [C++](./C++/zigzag-iterator.cpp) [Python](./Python/zigzag-iterator.py) | _O(n)_        | _O(1)_          | Medium           |📖||
 
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