gray-code——找规律

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 - 0
01 - 1
11 - 3
10 - 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.

n=0    0

n=1    0    0+1<<(1-1)

n=2    n=1: 0  1   1+1<<(2-1)   0+1<<(2-1)

找到规律，第n次增加的序列为n-1序列的倒序+（高位+1）形成。

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

     return res;
     }
 };