Python – Find all the strings that are substrings to the given list of strings

Given two lists, the task is to write a Python program to extract all the strings which are possible substring to any of strings in another list.

Example:

Input : test_list1 = [“Geeksforgeeks”, “best”, “for”, “geeks”], test_list2 = [“Geeks”, “win”, “or”, “learn”]

Output : [‘Geeks’, ‘or’]

Explanation : “Geeks” occurs in “Geeksforgeeks string as substring.

Input : test_list1 = [“geeksforgeeks”, “best”, “4”, “geeks”], test_list2 = [“Geeks”, “win”, “or”, “learn”]

Output : []

Explanation : No substrings found.

Method #1: Using list comprehension

In this, we perform task of using nested loop and testing using list comprehension, extracting the string if its part of any substring of other list.

The original list 1 is : ['Geeksforgeeks', 'best', 'for', 'geeks']
The original list 2 is : ['Geeks', 'win', 'or', 'learn']
Substrings Intersections : ['or', 'Geeks']

Time Complexity: O(n)
Auxiliary Space: O(1)

Method #2: Using any() + generator expression

In this, any() is used to check for substring matching in any of the strings from the string list to be matched in.

The original list 1 is : ['Geeksforgeeks', 'best', 'for', 'geeks']
The original list 2 is : ['Geeks', 'win', 'or', 'learn']
Substrings Intersections : ['Geeks', 'or']

Time Complexity: O(n²)

Space Complexity: O(n)

Method #3: Using find() method

The original list 1 is : ['Geeksforgeeks', 'best', 'for', 'geeks']
The original list 2 is : ['Geeks', 'win', 'or', 'learn']
Substrings Intersections : ['Geeks', 'or']

Time Complexity: O(n*n)
Auxiliary Space: O(n)

Method#4: Using Recursive method.

we define a recursive function substring_intersections() that takes in two lists of strings as input. It first checks if the second list list2 is empty. If it is, the function returns an empty list [].

If the second list list2 is not empty, the function checks if the first string in list2 is a substring of any element in the first list list1. If it is, it appends it to the result list res.

The function then recursively calls itself with the first element of list2 removed and the same first list list1. The result of this recursive call is concatenated with the result list res.

Finally, the function returns the result list res.

The original list 1 is : ['Geeksforgeeks', 'best', 'for', 'geeks']
The original list 2 is : ['Geeks', 'win', 'or', 'learn']
Substrings Intersections : ['Geeks', 'or']

The time complexity of this recursive approach is O(n * m * k) where n, m and k are the lengths of the two input lists and the maximum length of a string in the input lists.

 The space complexity is O(k) where k is the maximum length of a string in the input lists.

Method 5 :  use is the set intersection method. 

 steps for this approach:

Convert both test_list1 and test_list2 to sets, which removes any duplicate elements and makes checking for intersections faster.
Take the intersection of the two sets using the “&” operator.
Convert the resulting set back to a list.

The original list 1 is : ['Geeksforgeeks', 'best', 'for', 'geeks']
The original list 2 is : ['Geeks', 'win', 'or', 'learn']
Substrings Intersections : ['Geeks', 'or']

The time complexity of this approach is O(mnk), where m and n are the lengths of test_list1 and test_list2, respectively, and k is the maximum length of a substring in either list. 

The space complexity is O(k) for storing the res list.