Stone Game IX — C++

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

class Solution {
public:
    bool stoneGameIX(vector<int>& stones) {
        unordered_map<int, int> count;
        for (const auto& x : stones) {
            ++count[x % 3];
        }
        if (count[0] % 2 == 0) {
            // iff both counts are not zero, then alice takes the least one at first, the remains are deterministic for bob to lose:
            // - assumed count[1] is the least one:
            //   1(,1,2)*,2,(,2)* => bob loses
            //            ^
            // - assumed count[2] is the least one:
            //   2(,2,1)*,1,(,1)* => bob loses
            //            ^
            return count[1] && count[2];
        }
        // iff abs(count[1]-count[2]) >= 3, then alice takes the most one at first, the remains are deterministic for bob to lose:
        // - assumed count[1] is the most one
        //   1(,1,2)*,0,1(,2,1)*,1,(,1)* => bob loses
        //                       ^
        //   1(,1,2)*,1,0,1,(,1)* => bob loses
        //                ^
        // - assumed count[2] is the most one
        //   2(,2,1)*,0,2(,1,2)*,2,(,2)* => bob loses
        //                       ^
        //   2(,2,1)*,2,0,2,(,2)* => bob loses
        //                ^
        return abs(count[1] - count[2]) >= 3;
    }
};