# Time:  O(n)
# Space: O(n)

# On a table are N cards, with a positive integer printed on the front
# and back of each card (possibly different).
#
# We flip any number of cards, and after we choose one card.
#
# If the number X on the back of the chosen card is not on the front of
# any card, then this number X is good.
#
# What is the smallest number that is good?  If no number is good, output 0.
#
# Here, fronts[i] and backs[i] represent the number on the front and back of
# card i.
#
# A flip swaps the front and back numbers, so the value on the front is
# now on the back and vice versa.
#
# Example:
#
# Input: fronts = [1,2,4,4,7], backs = [1,3,4,1,3]
# Output: 2
# Explanation: If we flip the second card, the fronts are [1,3,4,4,7] and
# the backs are [1,2,4,1,3].
# We choose the second card, which has number 2 on the back,
# and it isn't on the front of any card, so 2 is good.
#
# Note:
# - 1 <= fronts.length == backs.length <= 1000.
# - 1 <= fronts[i] <= 2000.
# - 1 <= backs[i] <= 2000.

import itertools


class Solution(object):
    def flipgame(self, fronts, backs):
        """
        :type fronts: List[int]
        :type backs: List[int]
        :rtype: int
        """
        same = {n for i, n in enumerate(fronts) if n == backs[i]}
        result = float("inf")
        for n in itertools.chain(fronts, backs):
            if n not in same:
                result = min(result, n)
        return result if result < float("inf") else 0

Create card-flipping-game.py