LeetCode Gray Code

原题链接在这里：https://leetcode.com/problems/gray-code/

题目：

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

题解：

根据graycode的特性：在n增大1时，在原有list的末尾按照原有list的倒序每个数字首位digit加上一个1 加回到list中.

n = 0时，[0]

n = 1时，[0,1]

n = 2时，[00,01,11,10] 原有[0,1], 按照倒序[1,0], 首位加1变成[11,10]加回到原有的[0,1]后面, 就变成[0, 1, 11, 10]了.

n = 3时，[000,001,011,010,110,111,101,100]

Time Complexity: O(2^n). Space: O(1).不考虑res.

AC Java:

public class Solution {
    public List<Integer> grayCode(int n) {
        List<Integer> res = new ArrayList<Integer>();
        if(n<0){
            return res;
        }
        //when n = 0, return [0]
        res.add(0);
        for(int i = 0; i<n; i++){
            int len = res.size();
            //addNum 是要加的首位1
            int addNum = 1<<i;
            for(int j = len-1; j>=0; j--){
                res.add(addNum+res.get(j));
            }
        }
        return res;
    }
}