## 题目描述：

LeetCode 822. Card Flipping Game

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`.

## Python代码：

``````class Solution(object):
def flipgame(self, fronts, backs):
"""
:type fronts: List[int]
:type backs: List[int]
:rtype: int
"""
numbers = set(fronts + backs)
for n in sorted(numbers):
if all(f != n or b != n for f, b in zip(fronts, backs)):
return n
return 0
``````

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