89. 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.

 

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public IList<int> GrayCode(int n) {
        IList<int> res = new List<int>(){0};
        var visited = new HashSet<int>();
        visited.Add(0);
        return BackTracking(n,res,visited,0);
    }
    
    private IList<int> BackTracking(int n, IList<int> res, HashSet<int> visited, int cur )
    {
        if(res.Count() >=Math.Pow(2,n))
        {
            return res;
        }
        else
        {
            
            for(int i =0;i<n;i++)
            {
                int compare = 1 << i;
                int a = cur ^ compare;
                var temp = visited;
                if(visited.Add(a))
                {
                    res.Add(a);
                    var b =  BackTracking(n,res,visited,a);
                    if(b != null) return b;
                    res.RemoveAt(res.Count()-1);
                }
                visited = temp;
            }
        }
        return null;
    }