diff --git a/README.md b/README.md
index 1171072..c43bae4 100644
--- a/README.md
+++ b/README.md
@@ -114,6 +114,8 @@ Each challenge has two notebooks, a **challenge notebook** for you to solve and
 | Determine if a string is a rotation of another | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/rotation/rotation_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/rotation/rotation_solution.ipynb) |
 | Compress a string | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/compress/compress_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/compress/compress_solution.ipynb) |
 | Reverse characters in a string | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/reverse_string/reverse_string_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/reverse_string/reverse_string_solution.ipynb) |
+| Given two strings, find the single different char | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/str_diff/str_diff_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/str_diff/str_diff_solution.ipynb) |
+| Find two indices that sum to a specific value | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/two_sum/two_sum_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/two_sum/two_sum_solution.ipynb) |
 | Implement a hash table | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/hash_map/hash_map_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/hash_map/hash_map_solution.ipynb) |
 | Implement fizz buzz | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/fizz_buzz/fizz_buzz_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/arrays_strings/fizz_buzz/fizz_buzz_solution.ipynb) |
 | Find the first non-repeated character in a string | [Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md)│[Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md) |