[LeetCode-Algorithms-89] "Gray Code" (2017.12.26-WEEK17)

题目链接：Gray Code

• 题目描述：
  

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.

---

(1)思路：格雷码有一个小规律就是n位格雷码后分别再加0/1就有了n+1位格雷码。本来我想的是给后面加位数，但是发现实现起来不如直接往前加1方便。

举个例子，2位格雷码：

00
01
11
10

要变成3位格雷码，给最高位之前加1就好了，但是必须从后往前循环。

000
001
011
010
110
111
101
100

(2)代码：

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

(3)提交结果：