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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> ans;
        
        if (n == 0)
        {
            ans.push_back(0);
            return ans;
        }   
                
        int size = 1;
        for (int i = 0; i < n; ++i)
        {
            size *= 2;
        }
            
        //bool *f = new bool[size];
        //memset(f, 0, sizeof(int) * size);
        vector<bool> f;
    	f.resize(size);	
        ans.resize(size);
    	ans[0] = 0;
    	f[0] = true;
        DFS(ans, size, 1, f, 0, n);               
        //delete[] f;    
    	return ans;
    }
    
    bool DFS(vector<int> &ans, int total, int step, vector<bool> &f, int num, int N)
    {
        if (step == total)
        {
            return true;    
        }
        for (int i = 0; i < N; ++i)
        {
            int tmp = (num ^ (1 << i));
            if (!f[tmp])
            {
                f[tmp] = true;
                ans[step] = tmp;
                if (DFS(ans, total, step + 1, f, tmp, N))
                {
                    return true;
                }
                f[tmp] = false;
            }
        }
        return false;  
    }
};