def select_smallest(my_list, beg, end, i):
   if end - beg <= 1:
      return my_list[beg]
   pivot_val = start_partition(my_list, beg, end)

   k = pivot_val - beg + 1

   if i < k:
      return select_smallest(my_list, beg, pivot_val, i)
   elif i > k:
      return select_smallest(my_list, pivot_val + 1, end, i - k)

   return my_list[pivot_val]

def start_partition(my_list, beg, end):
   pivot_val = my_list[beg]
   i = beg + 1
   j = end - 1

   while True:
      while (i <= j and my_list[i] <= pivot_val):
         i = i + 1
      while (i <= j and my_list[j] >= pivot_val):
         j = j - 1

   if i <= j:
      my_list[i], my_list[j] = my_list[j], my_list[i]
   else:
      my_list[beg], my_list[j] = my_list[j], my_list[beg]
      return j

my_list = input('输入数字列表.. ')
my_list = my_list.split()
my_list = [int(x) for x in my_list]
i = int(input('输入i的值.. '))

ith_smallest = select_smallest(my_list, 0, len(my_list), i)
print('结果是{}。'.format(ith_smallest))

输入数字列表.. 43 12 67 89 99 0
输入i的值.. 3
结果是43。