Gray Code

Q:

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.

 

A:

仍然是个dp，想一想A[i-1]的每个元素已经是gray-code了，我们将A[i-1]从尾到头遍历并加上2^（i-1），这些元素都append在A[i-1]之后就是A[i]了。

class Solution {
 public:
  vector<int> grayCode(int n) {
    // Start typing your C/C++ solution below
    // DO NOT write int main() function
    vector<int> result; 
    result.push_back(0);
    for (int i = 0; i < n; ++i) {
      int org_size = result.size();
      for (int j = org_size - 1; j >= 0; --j) {
        result.push_back((1<<i) + result[j]);
      } 
    } 
    return result;
  } 
};