PracticeArrays

Flip the Token Order in Every Document

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Context

A text archive stores each document as a separate list of compact token labels. A cleanup stage needs to read every document from its final token back to its first token, while keeping documents separate. Build the transformation that reverses each document independently.

Problem

Given a collection of documents, where each document is represented by a sequence of token labels, reverse the token order inside every document. Keep the outer collection unchanged: preserve the number of documents, each document's length, every token, and every document boundary. An empty archive is valid, and an empty or one-token document must remain valid and require no special placeholder. Repeated token labels must stay repeated and simply appear in the opposite positions. Return the transformed collection.

Examples

Example 1

Input: documents = [["alpha", "beta", "gamma"], ["red", "red", "blue"], ["solo"]]

Output: [["gamma", "beta", "alpha"], ["blue", "red", "red"], ["solo"]]

Explanation: The first document changes from ["alpha", "beta", "gamma"] to ["gamma", "beta", "alpha"], the repeated labels in the second document remain repeated in reversed positions, and the one-token document remains ["solo"].

Example 2

Input: documents = [[], ["open", "ai"], [], ["token"]]

Output: [[], ["ai", "open"], [], ["token"]]

Explanation: The empty documents remain empty, ["open", "ai"] becomes ["ai", "open"], and the one-token document remains ["token"].

Example 3

Input: documents = [["a1", "b2"], ["cat", "dog", "cat"], ["x9", "y8", "z7"]]

Output: [["b2", "a1"], ["cat", "dog", "cat"], ["z7", "y8", "x9"]]

Explanation: The first and third documents reverse their token orders, while ["cat", "dog", "cat"] has the same displayed order because its outer tokens are equal.

Constraints

0 <= len(documents) <= 100
0 <= len(documents[i]) <= 100 for every i
1 <= len(documents[i][j]) <= 24 for every i, j
documents[i][j]: alphabet alnum lower
Types: documents is str[][]; result is str[][]

Function signature

def reverse_document_tokens(documents: list[list[str]]) -> list[list[str]]

documents list[list[str]]: A collection of documents, each represented by an ordered list of lowercase alphanumeric token labels.
returns list[list[str]]: The same collection shape, with the token sequence of each document reversed independently.

Adapted from MBPP problem task_889 (CC BY 4.0). Rewritten, extended and verified by Iksha.

Notes

The outer collection and every document must remain separate.
The transformation is about position, not alphabetical sorting.

Code

Saved

Checking environment…Python SandboxLn 1, Col 1

documents =