[leetcode][回溯] Gray Code

题目：

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.

递归实现：

<pre name="code" class="cpp">class Solution {public:    vector<int> grayCode(int n) {vector<int> res;res.push_back(0);if(n == 0) return res;vector<int> ret = grayCode(n - 1);int high = (1 << (n - 1));res = ret;for (int i = ret.size() - 1; i >= 0; --i){res.push_back(high + ret[i]);}return res;}};

非递归实现：

class Solution {public:    vector<int> grayCode(int n) {        vector<int> res;        res.push_back(0);        for(int i = 0; i < n; ++i){            int len = res.size();            int highest = 1 << i;            for(int j = len-1; j >= 0; --j){                res.push_back(highest | res[j]);            }        }        return res;    }};