格雷编码（思维）

题目：https://leetcode-cn.com/problems/gray-code/ 参考：https://leetcode-cn.com/problems/gray-code/solution/gray-code-jing-xiang-fan-she-fa-by-jyd/

class Solution {
public:
    vector grayCode(int n) {
        /*
        *镜像反射法
        *令G[n]表示阶数为n的格雷编码
        *那么G[n+1]等于G[n] + reverse(G[n])+高位1
        */
        vector ans;
        ans.push_back(0);
        int res = 1;
        for(int k = 1;k = 0;i--)
                ans.push_back(res+ans[i]);
            res