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

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.

就是把n位二进制数都转成十进制返回。代码：

    vector<int> grayCode(int n) {
        vector<int> result;
        if(!n) {
            result.push_back(0);
            return result;
        }
        bool* bits = (bool*)malloc(n);
        memset(bits,0,n);
        getGrayCode(result,n,0,bits);
        return result;
    }
    void getGrayCode(vector<int>& result,int n,int i,bool* bits){
        if(n==i){
            result.push_back(b2i(bits,n));
            return ;
        }
        getGrayCode(result,n,i+1,bits);
        bits[i]=!bits[i];
        getGrayCode(result,n,i+1,bits);
    }
    int b2i(bool* bits,int n){
        int result=0,c=0;
        for(int i=n-1;i>=0;i--){
            if(!c) c=1;
            else c*=2;
            if(bits[i]){
                result += c;
            }
        }
        return result;
    }

时间复杂度太大了，以为过不了，结果还是过了，都2ⁿ*n了。不过规模已经是指数级，没办法降啊。
后来看到这个解法，于是去wikipedia上面看了下，确实非常简单，下面的链接：
http://blog.csdn.net/doc_sgl/article/details/12251523