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77. Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

public IList<IList<int>> Combine(int n, int k) {        var res = new List<IList<int>>();        BackTracking(res,n,k,1,new List<int>());        return res;    }        private void BackTracking(IList<IList<int>> res, int n,int k, int sentinel, IList<int> cur)    {        if(k==0) res.Add(new List<int>(cur));        else        {            for(int i = sentinel; i <= n;i++ )            {                cur.Add(i);                BackTracking(res,n,k-1,i+1,cur);                cur.RemoveAt(cur.Count()-1);            }        }    }

 

77. Combinations