# 822. Card Flipping Game 翻转卡片游戏

## # 题目描述：

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. 1 <= fronts.length == backs.length <= 1000.
2. 1 <= fronts[i] <= 2000.
3. 1 <= backs[i] <= 2000.

## # 解题方法

``````class Solution:
def flipgame(self, fronts, backs):
"""
:type fronts: List[int]
:type backs: List[int]
:rtype: int
"""
s = set()
res = float('inf')
for f, b in zip(fronts, backs):
if f == b:
for f in fronts:
if f not in s:
res = min(res, f)
for b in backs:
if b not in s:
res = min(res, b)
return 0 if res == float('inf') else res
``````