二分查找，举一反三

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
        if guess >item:
            high = mid - 1
        else:
            low  = mid + 1
    return None

my_list = [1,3,5,7,9]
print (binary_search(my_list,3)) # => 1
print (binary_search(my_list,-1)) # => None

二分查找，是对数时间。会比线性时间少很多。

大O表示法