1927. Sum Game

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def sumGame(self, num):
        """
        :type num: str
        :rtype: bool
        """
        # (1) if both halfs have '?',
        #     alice will optimally choose 9 or 0 from one half to maximize or minimize the diff of both half sums,
        #     and bob will optimally choose the same number from the other half to minimize or maximize the diff of both half sums.
        #     in the end, it turns that only one half has '?' and the diff of both half sums is still the same as original
        # (2) if smaller half has no '?', then alice wins
        # (3) if smaller half has '?'
        #     (3.1) if cnt of '?' is odd, alice can choose the last number to make the diff of both half sums != 0, then alice wins
        #     (3.2) if cnt of '?' is even
        #           (3.2.1) if larger-smaller = cnt/2 * 9, bob can always make a pair of sum 9 no matter what alice chooses, then bob wins
        #           (3.2.2) if larger-smaller > cnt/2 * 9, alice can always choose 0 no matter what bob chooses, then alice wins
        #           (3.2.3) if larger-smaller < cnt/2 * 9, alice can always choose 9 no matter what bob chooses, then alice wins
        cnt = total = 0
        for i in xrange(len(num)):
            if num[i] == '?':
                cnt += (-1 if i < len(num)//2 else 1)
            else:
                total += (int(num[i]) if i < len(num)//2 else -int(num[i]))
        return True if cnt%2 else total != cnt//2*9

Solution from kamyu104/LeetCode-Solutions · MIT