$1.$List & Set $(50\%)$

We want to build a friend recommendation system. When two users who are unfamiliar with each other

know someone in common, we will provide recommendations so that they can get to know each other.

However, given that some people may be offended by specific individuals, we would like to avoid

unnecessary referrals, so please help these users refer some new friends. Remarks: $1.$ There is no situation

where A knows B$,$ but B does not know A$.2.$ Once you know each other, you can no longer recommend

them.

$1.$List & Set $(50\%)$

$$:

Input:

Know:

$1,A={[}^{\text{'}}{B}^{\text{'}}]$

$2,B={[}^{\text{'}}{A}^{\text{'}},C^{\text{'}},{?}^{\text{'}}{D}^{\text{'}}]$

$3,C={[}^{\text{'}}{B}^{\text{'}},{?}^{\text{'}}{E}^{\text{'}},F^{\text{'}}]$

Blacklist.:

$NA={[}^{\text{'}}{C}^{\text{'}},{?}^{\text{'}}{E}^{\text{'}}]$

$NB=[F^{\text{'}}]$

$NC=[A^{\text{'}},D^{\text{'}}]$