Flatten a Nested List

Source: Dev.to

Hey everyone! 👋
I know I’ve been a bit quiet lately. I actually came down with a pretty bad flu last week, which completely knocked me out. 🤒 That’s why I missed posting about the coding challenges. I’m finally feeling a bit better and ready to get back into the swing of things!

Problem Statement

The goal is to write a function that takes a list which might contain other lists (nested to any depth) and converts it into a single, one‑dimensional list.

Examples

flatten([[1, 2], [3, 4], [5, 6]])  # → [1, 2, 3, 4, 5, 6]

flatten([1, [2, 3], [[4, 5], 6]])  # → [1, 2, 3, 4, 5, 6]

Solution

Here is the Python implementation using recursion. Recursion is perfect here because we don’t know how deep the nesting goes!

def flatten(nested_list):
    """
    Flattens a nested list into a single-level list.
    """
    result = []
    for item in nested_list:
        if isinstance(item, list):
            flattened_sublist = flatten(item)  # Recursively flatten the sublist
            result.extend(flattened_sublist)
        else:
            result.append(item)
    return result

# Test cases
print(flatten([[1, 2], [3, 4], [5, 6]]))
# Output: [1, 2, 3, 4, 5, 6]

print(flatten([1, [2, 3], [[4, 5], 6]]))
# Output: [1, 2, 3, 4, 5, 6]

Explanation

• Function definition – def flatten(nested_list): defines a function that accepts the potentially nested list.
• Initialize result – result = [] creates an empty list to collect flattened elements.
• Iterate – for item in nested_list: loops over each element.
• Check for sublist – if isinstance(item, list): determines whether the current element is itself a list.
• Recursive call – flattened_sublist = flatten(item) flattens the sublist.
• Extend result – result.extend(flattened_sublist) adds the flattened elements to the main result list.
• Handle non‑list items – else: result.append(item) directly appends atomic elements.
• Return – return result yields the fully flattened list.

The beauty of this function is its flexibility. Thanks to recursion, it can handle a list nested 2 levels deep or 200 levels deep with the exact same logic!

Thanks for sticking with me while I recovered! I’ll be catching up on more challenges soon. Happy coding! 💻