Sum Game — C++

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

class Solution {
public:
    bool sumGame(string num) {
        // (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
        int cnt = 0, total = 0;
        for (int i = 0; i < size(num); ++i) {
            if (num[i] == '?') {
                cnt += (i < size(num) / 2) ? -1 : 1;
            } else {
                total += (i < size(num) / 2) ? num[i] - '0' : -(num[i] - '0');
            }
        }
        return cnt % 2 ? true : total != cnt / 2 * 9;
    }
};