LeetCode: Gray Code

LeetCode: 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.

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.

地址：https://oj.leetcode.com/problems/gray-code/

算法：假设我们已经完成了前面i位的Gray 码，那么前面i+1位Gray应该按如下构造，首先是第i+1位为零部分，其后的i位跟前面i位Gray码一致，然后紧接着是第i+1位为1的部分，其后的i位按前面i位Gray码的逆序排。假如，n=2Gray码为00 01 11 10，那么n=3的Gray码为000 001 011 010 110 111 101 100。代码：

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int> result;
        if(n < 0)   return result;
        result.push_back(0);
        if(n == 0)  return result;
        result.push_back(1);
        for(int i = 1; i < n; ++i){
            int len = result.size();
            int temp = 1 << i;
            for(int j = len-1; j >= 0; --j){
                result.push_back(result[j] + temp);
            }
        }
        return result;
    }
};