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01. Print List ItemsE02. Sum List NumbersE03. Count List ElementsE04. Find Maximum ValueE05. Filter Even NumbersE06. Square Each NumberE07. Reverse a String with a LoopE08. Compute Running TotalE09. Iterate With RangeE10. Repeat Until Condition MetE11. Find First NegativeE12. Skip Empty StringsE13. Index and Value with EnumerateE14. Pair Elements With ZipE15. Iterate In Reverse OrderE16. Count Vowels with a LoopM17. Count Item FrequenciesM18. Flatten One-Level ListM19. Remove Consecutive DuplicatesM20. Rotate List by N PositionsM21. Build Multiplication TableM22. Split CSV Row Into FieldsM23. Check Palindrome with LoopM24. Generate Pascal's TriangleH25. Implement Run-Length EncodingH

Problem No 19

Remove Consecutive Duplicates

Medium

≈ 13 minute session

Back to curriculumNext lesson

Summary

Return a new list with consecutive duplicate values collapsed to a single occurrence.

Problem statement

Given a list, produce a new list where runs of the same value that appear consecutively are collapsed into a single occurrence. Non-consecutive duplicates (the same value separated by different values) should be preserved. Implement remove_consecutive_duplicates(lst) that returns the cleaned list. The original list should not be modified.

Task

Learn to iterate through a sequence and build a result while comparing current and previous elements.

Examples

Simple example

Input

remove_consecutive_duplicates([1, 1, 2, 2, 1])

Output

[1, 2, 1]

Explanation

Consecutive runs of 1 and 2 are collapsed, but the later 1 is kept because it's not consecutive with the earlier 1.

Input format

A single argument: lst (a list of values).

Output format

A list with consecutive duplicates removed.

Constraints

You may assume the input is a list. Elements can be of any type that supports equality comparison. Aim for linear time O(n) and O(n) additional space for the output.

Samples

Sample input 0

remove_consecutive_duplicates(['a', 'a', 'b', 'a'])

Sample output 0

['a', 'b', 'a']

Explanation 0

Only consecutive 'a' at the start are collapsed; the later 'a' remains.

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