Solve a few String questions

Valid Anagram

This one is a classic: `sorted(a) == sorted(b)`

Group Anagrams

Using product of prime numbers to create a key for anagrams is much faster than
sorting the characters in each word. It is also satisfyingly simple.

Encode and Decode Strings

My initial implementation was clumsy and prone to fail for edge-cases. A more
elegant solution is using something like:

```python
def encode(words):
  return "".join("{}:{}".format(len(x), x) for x in words)
```

scratch/facebook/leetcode.org

@@ -111,9 +111,9 @@
    https://leetcode.com/problems/longest-repeating-character-replacement/
 ** TODO Minimum Window Substring
    https://leetcode.com/problems/minimum-window-substring/
-** TODO Valid Anagram
+** DONE Valid Anagram
    https://leetcode.com/problems/valid-anagram/
-** TODO Group Anagrams
+** DONE Group Anagrams
    https://leetcode.com/problems/group-anagrams/
 ** DONE Valid Parentheses
    https://leetcode.com/problems/valid-parentheses/
@@ -123,7 +123,7 @@
    https://leetcode.com/problems/longest-palindromic-substring/
 ** TODO Palindromic Substrings
    https://leetcode.com/problems/palindromic-substrings/
-** TODO Encode and Decode Strings (Leetcode Premium)
+** DONE Encode and Decode Strings (Leetcode Premium)
    https://leetcode.com/problems/encode-and-decode-strings/
 * Tree
 ** DONE Maximum Depth of Binary Tree