create factorial code and fix bug

[lucywowen]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0f001e7). Click here to learn what that means.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master       #1   +/-   ##
=========================================
  Coverage          ?   71.42%           
=========================================
  Files             ?        1           
  Lines             ?       14           
  Branches          ?        2           
=========================================
  Hits              ?       10           
  Misses            ?        3           
  Partials          ?        1

Impacted Files	Coverage Δ
factorial.py	71.42% <66.66%> (ø)

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