算法图解---二分查找

二分查找（binary-search）是一种算法，输入是一个有序的元素列表（仅当列表是有序的时候，二分查找才管用）。若查找的元素包含在列表中，二分查找返回其位置，否则返回null。
1.查找的范围
low = 0
high = len(list) -1
2.检查中间的元素
mid = (low + high)/2 # 如果（low + high）是奇数，python自动将mid向下圆整
guess =list[mid]
#猜的数字小了，修改low
if guess < item:
low = mid +1
#猜的数字大了，修改high
if guess > item:
high = mid -1
3. 完整代码如下:
def binary_search(list,item):
low = 0
high = len(list)-1

while low <= high:
    mid = int((low + high)/2)
    guess = list[mid]
    if guess == item:
        return  mid
    elif guess > item:
        high = mid -1
    else:
        low = mid +1
return  None

my_list= [1,3,5,7,9]
print(binary_search(my_list,3))
print(binary_search(my_list, -1))

D:\python\routin\python.exe E:/pychar/ab.py
1
None