[leetcode] Gray Code

he 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> grayCode(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<int> ret;
if(n>32)
return ret;
set<int> checked;
ret.push_back(0);
checked.insert(0);
unsigned int num=0;
unsigned int flag=1;
unsigned int direction=0;
bool nonum=true;
int i=0;
for( ; ; direction=!direction,nonum=true){
for(i=0 ; i<n ; i++){
if(direction && i!=0)
flag>>=1;
if(!direction && i!=0)
flag<<=1;
if(checked.count(num^flag))
continue;
else{
num^=flag;
ret.push_back(num);
checked.insert(num);
nonum=false;
}
}
if(nonum==true && i==n)
break;
}
return ret;
}
};

格雷码的问题，给出一个更一般的解：

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

简单明了