二分查找：分而治之

def binarysearch(alist, item):
    if len(alist) == 0:
        return False
    else:
        midpoint = len(alist)//2
        if alist[midpoint] == item:
            return True
        else:
            if item<alist[midpoint]:
                return binarysearch(alist[:midpoint-1], item)
            else:
                return binarysearch(alist[midpoint+1:], item)


a = [3, 5, 7, 10, 14, 16, 18, 20]
print(binarysearch(a, 6))