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);
            }
        }
    }