选择排序

## 选择排序需要执行n次时间为O_(n)的操作，所以总时间为O_(n²)

 1 def find_smallest(arr):
 2     """return the index of the smallest element"""
 3     smallest = arr[0]
 4     smallest_index = 0
 5     for i in range(0, len(arr)):
 6         if arr[i] < smallest:
 7             smallest = arr[i]
 8             smallest_index = i
 9     return smallest_index
10 
11 def selection_sort(arr):
12     sorted_arr = []
13     for i in range(0, len(arr)):
14         smallest_index = find_smallest(arr)
15         sorted_arr.append(arr.pop(smallest_index))
16     return sorted_arr
17 
18 print(selection_sort([2,3,4,5,7,2,21,-2,-4,-9]))
19 # [-9, -4, -2, 2, 2, 3, 4, 5, 7, 21]