464 Can I Win 我能赢吗

class Solution {
public:
    bool canIWin(int maxChoosableInteger, int desiredTotal)
    {
        if (maxChoosableInteger >= desiredTotal)
        {
            return true;
        }
        if (maxChoosableInteger * (maxChoosableInteger + 1) / 2 < desiredTotal) 
        {
            return false;
        }
        unordered_map<int, bool> m;
        return canWin(maxChoosableInteger, desiredTotal, 0, m);
    }
    bool canWin(int length, int total, int used, unordered_map<int, bool>& m) 
    {
        if (m.count(used))
        {
            return m[used];
        }
        for (int i = 0; i < length; ++i) 
        {
            int cur = (1 << i);
            if ((cur & used) == 0) 
            {
                if (total <= i + 1 || !canWin(length, total - (i + 1), cur | used, m)) 
                {
                    m[used] = true;
                    return true;
                }
            }
        }
        m[used] = false;
        return false;
    }
};