Exercise: character counting

• Given a string count how many time each character appears in the string.

Input: "abcax"
Output:
a: 2
b: 1
c: 1
x: 1

• First implement a function that does this in a single thread.
• Then create a threaded solution with a shared HashMap where each thread updates the shared HashMap.
• Then create a threaded solution with local HashMaps and then updating the central HashMap at the end of the thread.