LeetCode 89. Gray Code (Java版; Medium)

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LeetCode Top Interview Questions 89. Gray Code (Java版; Medium)

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.

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].

第一次做; 根据优秀题解的思想写写例子, 自己总结一下规律; 核心: 从后往前遍历list, 在每个元素前面加上1, 再保存回list

n=0时, [0]
n=1时, [0,1]
n=2时, [00,01,11,10]
n=3时, [000,001,011,010,110,111,101,100]

每一次保留上一次的结果, 然后从后往前遍历上一次的结果, 在每个元素前面加上1, 再存入list

class Solution {
    public List<Integer> grayCode(int n) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(0);
        if(n==0)
            return list;
        for(int i=1; i<=n; i++){
            for(int j=list.size()-1; j>=0; j--){
                list.add(list.get(j)|(1<<(i-1)));
            }
        }
        return list;
    }
}

​​LeetCode优秀题解​​

public List<Integer> grayCode(int n) {
    List<Integer> rs=new ArrayList<Integer>();
    rs.add(0);
    for(int i=0;i<n;i++){
        int size=rs.size();
        for(int k=size-1;k>=0;k--)
            rs.add(rs.get(k) | 1<<i);
    }
    return rs;
}