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01. Create a list of integers from 1 to nE02. Access elements by positive and negative indexE03. Slice a list to get first and last three itemsE04. Append an item and extend a listE05. Insert and remove elements in a listE06. Find element index or return -1E07. Count occurrences of a value in a listE08. Reverse a list in-place and by slicingE09. Sort a list of numbers ascending and descendingE10. Convert between lists and tuplesE11. Interleave two lists element by elementM12. Flatten a single-level list of listsM13. Filter a list using a predicateM14. Map a function over a listM15. Rotate a list by k positionsM16. Get unique elements preserving orderM17. Partition a list by a conditionM18. Replace a sublist slice with another listM19. Group equal consecutive values into sublistsH20. Reconstruct a list from value-count tuplesH

Problem No 16

Get unique elements preserving order

Medium

≈ 16 minute session

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Summary

Return the unique elements from a list while keeping their first-seen order.

Problem statement

Write a function unique_preserve(lst) that takes a list and returns a new list with only the first occurrence of each element, preserving the original order. Do not use sorting. The function should handle any hashable elements (numbers, strings, tuples) and must preserve the order in which distinct elements first appear.

Task

Learn how to remove duplicates from a list without changing the relative order of first occurrences.

Examples

Basic example

Input

unique_preserve([1, 2, 2, 3, 1])

Output

[1, 2, 3]

Explanation

1 appears first, then 2, then 3. Subsequent duplicates are ignored.

Input format

A single list (e.g., [1, 2, 2, 3]) passed to unique_preserve.

Output format

A list containing the first occurrence of each distinct element, in original order.

Constraints

You may assume elements are hashable. Do not change the input list in-place (return a new list). Aim for O(n) time and O(n) extra space.

Samples

Sample input 0

unique_preserve([5, 5, 6, 7, 6])

Sample output 0

[5, 6, 7]

Explanation 0

Keeps first 5, first 6, and first 7; ignores later duplicates.

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