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.

思路：假设以二进制为0的值做为格雷码的0，G表示格雷码，B表示二进制码，则，G(N) = (B(n)/2) XOR B(n)。

 1 class Solution { 2 public: 3     vector<int> grayCode( int n ) { 4         int size = 1 << n; 5         vector<int> gray_code( size ); 6         for( int i = 0; i < size; ++i ) { 7             gray_code[i] = ( i>>1 ) ^ i; 8         } 9         return gray_code;10     }11 };