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.
class Solution:
    def flipgame(self, fronts: List[int], backs: List[int]) -> int:
        cands = [0]*2001
        blacklist = [0]*2001
        for f, b in zip(fronts, backs):
            if f == b:
                blacklist[f] = 1
                cands[b] = 0
            else:
                if not blacklist[f]:
                    cands[f] = 1
                if not blacklist[b]:
                    cands[b] = 1
                    
        for idx in range(1, 2001):
            if cands[idx]:
                return idx
        return 0
class Solution:
    def flipgame(self, fronts: List[int], backs: List[int]) -> int:
        cands = [0]*2001
        blacklist = [0]*2001
        for f, b in zip(fronts, backs):
            if f == b:
                blacklist[f] = 1
                cands[b] = 0
            else:
                if not blacklist[f]:
                    cands[f] = 1
                if not blacklist[b]:
                    cands[b] = 1
                    
        for idx in range(1, 2001):
            if cands[idx]:
                return idx
        return 0