找出从自然数 1, 2, …, n 中任取r个数的所有组合, 递归与非递归算法.
     例子:  n = 5     1  2  3  4  5
            r = 3                    
                                      5  4  3
                                      5  4  2
                                      5  4  1
                                      5  3  2
                                      5  3  1
                                      5  2  1
                                      4  3  2
                                      4  3  1
                                      4  2  1
                                      3  2  1