20 Sep 2017:modify the README.md

Jack-Cherish · Jack-Cherish · commit e2c02cbadf31 · 2017-09-20T10:26:00.000+08:00
diff --git a/README.md b/README.md
@@ -60,9 +60,12 @@
 | 551  |      *     | Student Attendance Record I              | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Easy/551.%20Student%20Attendance%20Record%20I/Student%20Attendance%20Record%20I.py "悬停显示") | no | no |
 | 557  |      *     | Reverse Words in a String III            | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Easy/557.Reverse%20Words%20in%20a%20String%20III/Reverse%20Words%20in%20a%20String%20III.py "悬停显示") | [C++](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Easy/557.Reverse%20Words%20in%20a%20String%20III/Reverse%20Words%20in%20a%20String%20III.cpp "悬停显示") | [博客思路讲解](http://blog.csdn.net/c406495762/article/details/73457057 "悬停显示") |
 | 657  |    Easy    | Judge Route Circle                       | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Easy/657.Judge%20Route%20Circle/Judge%20Route%20Circle.py "悬停显示") | no | no |
-| 17   |    Medium   | Letter Combinations of a Phone Number        | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/17.Letter%20Combinations%20of%20a%20Phone%20Number.md "悬停显示") | no | [思路讲解](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/17.Letter%20Combinations%20of%20a%20Phone%20Number.md "悬停显示") |
+| 5   |    Medium   | Longest Palindromic Substring            | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/5.Longest%20Palindromic%20Substring.md "悬停显示") | no | [思路讲解](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/5.Longest%20Palindromic%20Substring.md "悬停显示") |
+| 17   |       *        | Letter Combinations of a Phone Number        | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/17.Letter%20Combinations%20of%20a%20Phone%20Number.md "悬停显示") | no | [思路讲解](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/17.Letter%20Combinations%20of%20a%20Phone%20Number.md "悬停显示") |
 | 22   |    Medium   | Generate Parentheses                | [Python](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/22.Generate%20Parentheses.md "悬停显示") | no | [思路讲解](https://github.com/Jack-Cherish/LeetCode/blob/master/String/Medium/22.Generate%20Parentheses.md "悬停显示") |
 
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 ### Tree(树)
 
 | ID   | Difficulty |                  Title                   |                  Python                  |                   C++                    |                   Blog                   |
