Given two integers

*n*and*k*, return all possible combinations of*k*numbers out of 1 ...*n*.
For example,

If

If

*n*= 4 and*k*= 2, a solution is:[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]

Solution: a classical DFS problem, pay attention to the format of solve this question which can be apply to many other DFS problems