Optimize flattening for O(1) space

Question

Given a nested list of integers, implement a function that flattens the list into a single list of integers while optimizing for O(1) space complexity. The input will be a list that can contain integers or other nested lists, and the output should be a single flattened list of integers. Ensure that the solution does not use any additional data structures that grow with the input size.

Codemia · Accepted Answer

To flatten a nested list of integers while maintaining O(1) space complexity, we need to explore the concept of in-place modification. This problem can be approached using a depth-first search (DFS) strategy without utilizing additional data structures that grow with the input size. The key here is to traverse the nested structure recursively or iteratively, ensuring that we directly manipulate the original list to achieve the flattening effect. By carefully managing the indices and positions, we can achieve the flattening without requiring extra space.

1. Initialize a pointer to the start of the list. 
2. Iterate through the list using a while loop. 
3. If an element is an integer, move the pointer forward. 
4. If an element is a list, we need to insert its contents into the main list. We can achieve this by moving the contents of the nested list to the position of the current pointer, effectively shifting the remaining elements. 
5. Continue this process until the end of the list is reached.

**Example:** Given the input `nested_list = [1, [2, 3], 4, [5, [6]]]`, the process would look like: 
- Start with `1`, move pointer to next. 
- Encounter `[2, 3]`, replace it with `2, 3` and shift remaining elements. 
- Continue until all elements are processed.

Here is the Python code that implements the above logic:

```python
def flatten(nested_list):
    index = 0
    while index < len(nested_list):
        if isinstance(nested_list[index], list):
            # Insert elements of the nested list into the main list
            nested_list[index:index + 1] = nested_list[index]
        else:
            index += 1  # Move to next element if it's an integer
    return nested_list
```
This implementation modifies the original `nested_list` in place and flattens it.

The time complexity of this solution is O(n), where n is the total number of integers in the nested structure. Each integer is processed exactly once. The space complexity is O(1) since we are not using any additional data structures that grow with the input size; we're modifying the list in place. This directly aligns with the problem's requirement for O(1) space optimization.

1. **Empty List:** Input `[]` should return `[]`. 
2. **All Integers:** Input `[1, 2, 3]` should return `[1, 2, 3]`. 
3. **Deeply Nested Lists:** Input `[[[[1], 2], 3], 4]` should return `[1, 2, 3, 4]`. 
4. **Invalid Input:** Input `[1, 'a', [2]]` should be handled gracefully; the function can either raise an error or skip non-integer elements. Handling these edge cases ensures robustness in the solution.

Optimize flattening for O(1) space

Spotify

What the Interviewer Expects

Key Topics to Cover

How to Approach This

Possible Follow-up Questions

Sharpen Your Skills on Codemia

Sample Answer

Problem Analysis

Approach

Submit Your Answer

Spotify Software Engineer Interview Guide

Related Questions

Binary Search on graph

Count kth largest element in array

Find connected components in string

Merge Three Sorted strings

Binary Search on string