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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 - 001 - 111 - 310 - 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;    }

 

89. Gray Code