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.

 

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。

1 class Solution { 2 public: 3     vector
     
       grayCode(int n) { 4         vector
      
        res; 5         if(n<0) return res; 6         res.push_back(0); 7         for(int i=1;i<=n;i++){ 8             int size=res.size(); 9                 for(int j=size-1;j>=0;j--){10                     int tmp=1<<(i-1);11                     tmp|=res[j];12                     res.push_back(tmp);13                 }14             }15       16     return res;17     }18 };