Why do I get "None" as output for a function? Also, how to make a function run for both strings and numbers?

Question

The question is :

Write a function called merge that takes two already sorted lists of possibly different lengths, and merges them into a single sorted list, without using the inbuilt sort function

I tried :

list1=(input("Enter the elements of list :")).split()
list2=(input("Enter the elements of list :")).split()

merged_list,sorted_list=(list1+list2),[]

#Without using sort()method 

def merge(list1,list2):

    global merged_list,sorted_list

    if len(merged_list)==0 : return sorted_list

    sorted_list.append(min(merged_list))

    merged_list.remove(min(merged_list))

    merge(list1,list2)

print(merge(list1,list2))

This gives output as "None", I don't know why. Also, it works well for alphabetic strings, but doesn't work for numerics, For example, if you give [02,1,0123] ,it returns [0123,02,1]. What should I change to make it work for both strings and numbers?

Answer 1

This isn't exactly what you are asking, but I'll add this as an example of:

1. returning from the recursive calls
2. avoiding the unnecessary global variable

:

def merge(list1,list2):
    # base case: one or both are empty lists
    if len(list1) == 0 or len(list2) == 0:
        return list1 + list2
    
    if list1[0] < list2[0]:
        return [list1[0]] + merge(list1[1:], list2)
    else:
        return [list2[0]] + merge(list1, list2[1:])

    
list1 = [1, 4, 6, 8, 14, 22]
list2 = [1, 2, 5, 9, 10, 15, 18, 20]

merge(list1,list2)
# [1, 1, 2, 4, 5, 6, 8, 9, 10, 14, 15, 18, 20, 22]