恰好y总讲过

https://www.acwing.com/video/554/

之后就把字符串转成数字就好了

class Solution {
public:
typedef unsigned long long ULL;
    vector<int> grayCode(int n) {  
        if(n==0) return {0};
        vector<int> res;
        for(int i=0;i<1<<n;i++){
            string ans=f(n,i);
            res.push_back(change(ans,n));
        }
        return res;
    }
    string f(ULL n, ULL k)
    {
    if (!n) return "";

    if (k < (1ull << n - 1)) return "0" + f(n - 1, k);
    ULL t = (1ull << n) - 1;
    if (n == 64) t = -1;
    return "1" + f(n - 1, t - k);
    }
   int change(string s,int n){
        int a=0;
        for(int i=0;i<n;i++){
            if((s[i]-'0')&1){
                a+=1<<(n-i-1);
            }
        }
        return a;
    }
};