Python Program to Flatten a Nested List using Recursion

Given a nested list, the task is to write a python program to flatten a nested list using recursion.

Examples:

Input: [[8, 9], [10, 11, ‘geeks’], [13]]

Output: [8, 9, 10, 11, ‘geeks’, 13]

Input: [[‘A’, ‘B’, ‘C’], [‘D’, ‘E’, ‘F’]]

Output: [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’]

Step-by-step Approach:

• Firstly, we try to initialize a variable into the linked list.
• Then, our next task is to pass our list as an argument to a recursive function for flattening the list.
• In that recursive function, if we find the list as empty then we return the list.
• Else, we call the function in recursive form along with its sublists as parameters until the list gets flattened.
• Then finally, we will print the flattened list as output.

Below are some python programs based on the above approach:

Example 1:

Python3

# Python program to flatten a nested list

# explicit function to flatten a
# nested list
def flattenList(nestedList):

    # check if list is empty
    if not(bool(nestedList)):
        return nestedList

     # to check instance of list is empty or not
    if isinstance(nestedList[0], list):

        # call function with sublist as argument
        return flattenList(*nestedList[:1]) + flattenList(nestedList[1:])

    # call function with sublist as argument
    return nestedList[:1] + flattenList(nestedList[1:])


# Driver Code
nestedList = [[8, 9], [10, 11, 'geeks'], [13]]
print('Nested List:\n', nestedList)

print("Flattened List:\n", flattenList(nestedList))

Output:

Nested List:
 [[8, 9], [10, 11, 'geeks'], [13]]
Flattened List:
 [8, 9, 10, 11, 'geeks', 13]

the time complexity can be O(n^2), where n is the total number of elements in the list. 

 the space complexity can be O(n), where n is the total number of elements in the list. 

Example 2:

Python3

# Python program to flatten a nested list

# explicit function to flatten a
# nested list
def flattenList(nestedList):

    # check if list is empty
    if not(bool(nestedList)):
        return nestedList

     # to check instance of list is empty or not
    if isinstance(nestedList[0], list):

        # call function with sublist as argument
        return flattenList(*nestedList[:1]) + flattenList(nestedList[1:])

    # call function with sublist as argument
    return nestedList[:1] + flattenList(nestedList[1:])


# Driver Code
nestedList = [['A', 'B', 'C'], ['D', 'E', 'F']]
print('Nested List:\n', nestedList)

print("Flattened List:\n", flattenList(nestedList))

Output:

Nested List:
 [['A', 'B', 'C'], ['D', 'E', 'F']]
Flattened List:
 ['A', 'B', 'C', 'D', 'E', 'F']

The time complexity of the provided program is O(n), where n is the total number of elements in the input nested list. 

The auxiliary space complexity of the program is also O(n), where n is the total number of elements in the input nested list. 

Example 3:

Python3

# Python program to flatten a nested list

# explicit function to flatten a
# nested list
def flattenList(nestedList):

    # check if list is empty
    if not(bool(nestedList)):
        return nestedList

     # to check instance of list is empty or not
    if isinstance(nestedList[0], list):

        # call function with sublist as argument
        return flattenList(*nestedList[:1]) + flattenList(nestedList[1:])

    # call function with sublist as argument
    return nestedList[:1] + flattenList(nestedList[1:])


# Driver Code
nestedList = [[1], [2], [3], [4], [5]]
print('Nested List:\n', nestedList)

print("Flattened List:\n", flattenList(nestedList))

Output:

Nested List:
 [[1], [2], [3], [4], [5]]
Flattened List:
 [1, 2, 3, 4, 5]