算法學習：二分法

 1 def binary_search(list1,item):
 2     low = 0
 3     high = len(list1)-1
 4 
 5     while low <= high:       #只要范圍沒有縮小到只包含一個元素
 6         mid = (low+high)//2  #檢查中間元素
 7         guess = list1[mid]
 8         if guess == item:
 9             return mid       #返回索引
10         if guess > item:
11             high = mid -1
12         if guess < item:
13             low = mid + 1
14             
15     return None
16 
17 my_list = [1,3,5,7,9]
18 
19 print(binary_search(my_list,7))
20 print(binary_search(my_list,3))
21 print(binary_search(my_list,-1))