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.

Example 1:

Input: 2
Output: [0,1,3,2]
Explanation:
00 - 0
01 - 1
11 - 3
10 - 2

For a given n
 
, a gray code sequence may not be uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence.

00 - 0
10 - 2
11 - 3
01 - 1

Example 2:

Input: 0
Output: [0]
Explanation: We define the gray code sequence to begin with 0.
             A gray code sequence of n has size = 2n, which for n
 
 = 0 the size is 20 = 1.
             Therefore, for n = 0 the gray code sequence is [0].

題意

怎麽說呢，很腦殘

就在這道題的世界觀裏，一個二進制數只有其中一個位跟另一個數不一樣，這兩個數就可以成為相鄰關系

給位數求順序集合

題解

那就遞歸一下，每次遞歸都把n-1的結果反著遍歷一遍，可以保證是相鄰關系

 1 class Solution {
 2 public:
 3     vector<int> grayCode(int n) {
 4         vector<int
 
>ans;
 5         if (n == 0) {
 6             ans.push_back(0);
 7             return ans;
 8         }
 9         ans = grayCode(n - 1);
10         int m = ans.size(), pls = (1 << (n - 1));
11         for (int i = m - 1; i >= 0; i--) 
12             ans.push_back(pls + ans[i]);
13         return ans;
14     }
15 };

19.2.23 [LeetCode 89] Gray Code