19.2.23 [LeetCode 89] Gray Code

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].

题意

怎么说呢，很脑残

就在这道题的世界观里，一个二进制数只有其中一个位跟另一个数不一样，这两个数就可以成为相邻关系

给位数求顺序集合

题解

那就递归一下，每次递归都把n-1的结果反着遍历一遍，可以保证是相邻关系

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int>ans;
        if (n == 0) {
            ans.push_back(0);
            return ans;
        }
        ans = grayCode(n - 1);
        int m = ans.size(), pls = (1 << (n - 1));
        for (int i = m - 1; i >= 0; i--) 
            ans.push_back(pls + ans[i]);
        return ans;
    }
};