File: truncate-sentence/solution.py

Date: 2026-06-06

Time: 19:31

truncate-sentence/solution.py

Purpose

This file implements the solution to LeetCode 1816 — Truncate Sentence. It owns exactly one responsibility: given a sentence string and an integer k, return the first k words of that sentence as a single space-separated string.

Key Components

Solution.truncateSentence(self, s: str, k: int) -> str — The sole method. It splits the input on whitespace, slices the first k elements, and joins them back with spaces.

The implementation is a single expression: " ".join(s.split()[:k]). No intermediate state, no mutation.

Patterns

• One-liner idiom: The split-slice-join chain is the canonical Python pattern for "take first N words." It avoids manual index tracking or counting spaces.
• LeetCode Solution class convention: Every solution in this repo wraps its logic in a Solution class with a specifically-named method matching the LeetCode submission interface.

Dependencies

Imports: None — uses only Python builtins (str.split, str.join, list slicing).

Imported by: The "Imported By" list in the prompt is misleading — those are test files from *other* problems that happen to share a common test harness import pattern (likely importing Solution from their own solution.py). The actual consumer is truncate-sentence/test_solution.py.

Flow

1. s.split() tokenizes the sentence on any whitespace, producing a list[str].

2. [:k] slices the first k words. If k >= len(words), it returns all words (no error).

3. " ".join(...) reassembles the words with single spaces.

The entire method is a pure function — no side effects, no state.

Invariants

• The problem guarantees s contains only letters and spaces, with no leading/trailing spaces and no double spaces. Because of this, s.split() and s.split(" ") behave identically here — but split() (no argument) is the safer choice since it handles edge cases with multiple spaces gracefully.
• 1 <= k <= number of words in s per the problem constraints, so the slice never produces an empty list.

Error Handling

None. The method trusts its inputs per LeetCode's guarantees. If k exceeds the word count, Python's slice semantics silently return all available words — which is correct behavior, not a bug.

Topics to Explore

• [file] truncate-sentence/test_solution.py — See how edge cases (k equals word count, single word) are tested
• [file] truncate-sentence/plan.md — The problem analysis and approach reasoning before implementation
• [file] truncate-sentence/review.md — Post-implementation review notes, likely covering complexity and alternatives
• [general] split-vs-split-delimiter — Why s.split() (no args) differs from s.split(" ") in handling consecutive whitespace, and which LeetCode problems expose that distinction

Beliefs

• truncate-sentence-pure-function — truncateSentence is a pure function with no side effects, no instance state, and deterministic output for any given input
• truncate-sentence-no-imports — The solution uses zero imports; it relies entirely on Python builtin string and list operations
• truncate-sentence-safe-overslice — Passing k greater than the word count returns the full sentence without error, due to Python slice semantics
• truncate-sentence-single-expression — The entire solution body is a single return expression using the split-slice-join chain