Subfactorial, a recursive approach

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In how many ways are there that we can do gift exchanges so that no one gets his/hers own gift back? Watch and find out this "derangement" concept! #derangement #ChristmasGift

Check out !0,    • 0!=1 and !0=?  
Zero subfactorial

Derangement, recursive formula of subfactorial, https://brilliant.org/wiki/derangements/

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