Approach #1: Hash Set [Accepted]
Intuition
If a card has the same value x
on the front and back, it is impossible to win with x
. Otherwise, it has two different values, and if we win with x
, we can put x
face down on the rest of the cards.
Algorithm
Remember all values same
that occur twice on a single card. Then for every value x
on any card that isn't in same
, x
is a candidate answer. If we have no candidate answers, the final answer is zero.
Complexity Analysis

Time Complexity: , where is the length of
fronts
(andbacks
). We scan through the arrays. 
Space Complexity: .
Analysis written by: @awice.