leetcode #89 in cpp

The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 001 - 111 - 310 - 2

Note:
For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

Solution:

https://en.wikipedia.org/wiki/Gray_code

Check the section of constructing an n-bit gray code.

Code:

class Solution {public:    vector<int> grayCode(int n) {        if(n==0) return vector<int>{0};                vector<int> res;        vector<int> codes = grayCode(n-1);        //append 0        for(int i = 0; i < codes.size(); i ++)            res.push_back(codes[i] + (0<<(n-1)));        //append 1        for(int i = codes.size()-1; i >=0; i--){            res.push_back(codes[i] + (1<<(n-1)));        }                return res;    }        };