Google's Guard Game

Google's Guard game
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You're being held in a guarded room in Dr. Boolean's mad science lab. How can you escape?


Beta Rabbit, a fellow captive and rabbit, notices a chance. The lab guards are always exceptionally bored, and have made up a game for themselves. They take the file numbers from the various specimens in the Professors lab, and see who can turn them into single digit numbers the fastest.

"I've observed them closely," Beta says. "For any number, the way they do this is by taking each digit in the number and adding them all together, repeating with the new sum until the result is a single digit. For example, when a guard picks up the medical file for Rabbit #1235, she would first add those digits together to get 11, then add those together to get 2, her final sum."

See if you can short circuit this game entirely with a clever program, thus driving the guards back to severe boredom. That way they will fall asleep and allow you to sneak out!

Write a function answer(x), which when given a number x, returns the final digit resulting from performing the above described repeated sum process on x.

x will be 0 or greater, and less than 2^31 -1 (or 2147483647), and the answer should be 0 or greater, and a single integer digit.

Languages
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To provide a Python solution, edit solution.py
To provide a Java solution, edit solution.java

Test cases
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Inputs:
    (long) x = 13
Output:
    (int) 4

Inputs:
    (long) x = 1235
Output:
    (int) 2

Use verify [file] to test your solution and see how it does. When you are finished editing your code, use submit [file] to submit your answer. If your solution passes the test cases, it will be removed from your home folder.