I. recursive function

1) definition

• Calling function itself in function is recursion.
• The maximum depth of recursion in python is 1000, but the actual depth is less than 1000

def func():
    print("-----func-----")
    func()

func()

2) application

• You can use recursion to traverse various tree structures, such as folder system: you can use recursion to traverse all files in the folder

import os


def func(filepath, n):
    files_list = os.listdir(filepath)  # Get all files in the current folder
    for file in files_list:
        file_d = os.path.join(filepath, file)  # Real path of splicing files
        if os.path.isdir(file_d):  # Recursive entry to determine whether the file is a folder
            print("\t"*n, file)
            func(file_d, n+1)  #
        else:
            print("\t"*n, file)  # Recursive export

2. Binary search

• Advantages: half of the data can be removed each time, and the search efficiency is high
• Requirement: the search sequence must be an ordered sequence

1) non recursive algorithm

a) use index

# Let the user enter a number n. Judge this n Whether it appears in lst in
lst = [4, 56, 178, 253, 625, 1475, 2580, 3574, 15963]

left = 0
right = len(lst) - 1

num = int(input("Please enter a number n: "))
while left <= right:
    mid = (left + right) // 2
    if lst[mid] > num:
        right = mid - 1
    elif lst[mid] < num:
        left = mid + 1
    else:
        print("This number is in lst in")
        break
else:
    print("This number is not in lst in")

2) recursive algorithm

a) use index

# Let the user enter a number n. Judge this n Whether it appears in lst in
lst = [4, 56, 178, 253, 625, 1475, 2580, 3574, 15963]


def binary_search(lst, num, left, right):
    if left > right:
        return False
    mid = (left + right) // 2
    if lst[mid] > num:
        right = mid - 1
        return binary_search(lst, num, left, right)
    elif lst[mid] < num:
        left = mid + 1
        return binary_search(lst, num, left, right)
    else:return True

num = int(input("Please enter a number n: "))
ret = binary_search(lst, num, 0, len(lst)-1)
print(ret)

b) switch list

# Let the user enter a number n. Judge this n Whether it appears in lst in
lst = [4, 56, 178, 253, 625, 1475, 2580, 3574, 15963]


def binary_search(lst, num):
    if len(lst) == 0:
        return False
    mid = (len(lst) - 1) // 2
    if num > lst[mid]:
        return binary_search(lst[mid+1:], num)
    elif num < lst[mid]:
        return binary_search(lst[:mid], num)
    else:
        print("This number is in lst in")
        return True


num = int(input("Please enter a number n: "))

ret = binary_search(lst, num)
print(ret)