游游的问号替换

题目链接 ：C-游游的问号替换_牛客周赛 Round 15 (nowcoder.com)

 

 可以当成构造题目做， 长度最长只有1000， 并且每次dfs的正确答案只有一两个， 所以时间跑的很快

int main()
{
    string s; cin >> s;
    int n = s.size();
    auto dfs = [&](auto &&dfs, string str, int s) -> string 
    {
        if(s >= n) return str;
        char cp = str[s];
        for(int i = 0; i <= 2; i ++)
        {
            if(cp != '?' && cp - '0' != i) continue;//非？时跳过构造相同情况     
            
            str[s] = (char)(i + '0');//构造？等于0.1.2的情况
            
            if(s > 0 && str[s] == str[s - 1]) continue;
            
            if(s <= 1)
            {
                string res = dfs(dfs, str, s + 1);
                if(res != "null") return res;
            }
            else
            {
                int sum = (str[s - 2] - '0' * 9) + (str[s - 1] - ' 0') * 3 + (str[s] - '0');
                if(sum & 1) continue;
                string res = dfs(dfs, str, s + 1);
                if(res != "null") return res;
            }
        }
        return "null";
    };
    string res = dfs(dfs, s, 0);
    cout << (res == "null" ? "-1" : res) << endl;
    return 0;
}