Define a recursive function named merge chars, it is passed two str arguments, in which all the characters appear in alphabetic order; it returns a str that contains all the characters in its argument strings, also appearing in alphabetic order.

def merge_chars(str1, str2):

    if not str1 and not str2:

         return ''

    if str1 and (not str2 or str1[0] <= str2[0]):

         return str1[0] + merge_chars(str1[1:], str2)

    else:

         return str2[0] + merge_chars(str1, str2[1:])

Note: It's written in Python3

Explanation:

This line defines the function

def merge_chars(str1, str2):

This checks for empty strings. If yes, it return an empty string

    if not str1 and not str2:

         return ''

The following merge and sorts both strings, recursively

This checks if only one of the strings is not empty

    if str1 and (not str2 or str1[0] <= str2[0]):

         return str1[0] + merge_chars(str1[1:], str2)

This checks if both strings are not empty

    else:

         return str2[0] + merge_chars(str1, str2[1:])

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