4.1请编写前述sum函数的代码。

def diy_sum(arr):if not arr:    return 0elif len(arr) == 1:    return arr[0]else:    return arr.pop(0) + diy_sum(arr)arr = [1, 2, 3]print(diy_sum(arr))

4.2编写一个递归函数来计算列表包含的元素数。

def count_elements(list):if not list:    return Noneelif len(list) == 1:    return 1else:    return 1 + count_elements(list[1:])list = [1, 2, 3]print(count_elements(list))

4.3找出列表中最大的数字

def bigger(int1, int2):

if int1 >= int2:    return int1else:    return int2

def find_biggest(list):

if not list:    return Noneelif len(list) == 1:    return list[0]elif len(list) == 2:    return bigger(list[0], list[1])else:    return bigger(list[0], find_biggest(list[1:]))list = [1, 5, 3, 2]print(find_biggest(list))

4.4 还记得第1章介绍的二分查找吗？它也是一种分而治之算法。你能找出二分查找算法的基线条件和递归条件吗？

def binary_search_basic(list, target, low, high):if low > high:    return Noneelse:    mid = int((low + high) / 2 + 0.5)    guess = list[mid]    if guess == target:        return mid    elif guess > target:        high = mid - 1        return binary_search_basic(list, target, low, high)    else:        low = mid + 1        return binary_search_basic(list, target, low, high)def binary_search_dc(list, target):    return binary_search_basic(list, target, 0, len(list) - 1)list = [1, 2, 3, 4, 5]target = 3print(binary_search_dc(list, target))

参考：
https://blog.csdn.net/XIAOZHI...