Update README.md

Author: sandeep-exe

Practice Problems/Strings/README.md

@@ -5,6 +5,7 @@ All questions added here are taken from various sites like GeeksForGeeks, LeetCo
 - [Buddy String](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Buddy%20String)
 - [Hamming Distance](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Hamming%20Distance)
 - [Help Me Play](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Help%20Me%20Play)
+- [Longest Substring without Repeating Char](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Longest%20Substring%20without%20Repeating%20Char)
 - [Reverse In Parentheses](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Reverse%20In%20Parentheses)
 - [Valid Palidrome](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Valid%20Palidrome)
 - [Vaild Word Abbreviation](https://github.com/srsandy/Data-Structures-and-Algorithms-in-Java-2nd-Edition-by-Robert-Lafore/tree/master/Practice%20Problems/Strings/Valid%20Word%20Abbreviation)