leetcode笔记：Gray Code（2016腾讯软件开发笔试题）

一.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.

class Solution
{
public:
vector<int> buildGrayCode(int n)
{
}

int ans(int n)
{
//每一位都 i & (i -1)
vector<int> vec;
int x = 1 << n;
for(int i=0; i<x; i++)
{
vec.push_back(i & (i>>1));
cout << vec[i] << ' ';
}
return vec;
}

//可以改成递归
int recur_ans(int n)
{
vector<int> vec;
if(n < 0 ) return vec;
if(n == 1) {
vec.push_back(0);
vec.push_back(1);
return vec;
}
for(int i=2; i<n; i++)
{
int highbit = 1 << (i - 1);
for(int j = vec.size() -1; j>=0; j--)
{
vec.push_back(highbit|j);
}
}
return vec;
}

vector<int> recur_ans2(int n)
{
if(n ==0)
{
vector<int> vec;
vec.push_back(0);
return vec;
}else{
vector<int> res = recur_ans2(n-1);
int len = res.size();
int highbit = 1 << (n-1);
for(int i=len-1; i>=0; i--)
{
res.push_back(res[i] | highbit );
}
return res;
}
}
};