[LeetCode] Gray Code

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> grayCode(int n) {
        vector<int> res;
        res.push_back(0);
        if(n<1)//input:0,expected output:[0]
            return res;
        int index,diffValue;
        for(int i=0;i<n;i++){
           diffValue = (1<<i);
           index = res.size();
           while(index){ 
               index--;
               res.push_back(res[index]+diffValue);
               
           }//end while
           
        }//end for

     return res;

    }
};