Date: 2026-06-06
Time: 19:03
This file solves LeetCode 2553 — Separate the Digits in an Array. Given a list of positive integers, it returns a flat list of their individual digits in order. For example, [13, 25, 83, 77] becomes [1, 3, 2, 5, 8, 3, 7, 7].
It's one of ~500 problem solutions in the leetcode-implementations repo, each following the same structure: a solution.py with the core logic, a test_solution.py, and optional plan.md/review.md.
separate_digits(nums: list[int]) -> list[int] — The sole export. Takes a list of positive integers and returns a list of single digits.
The implementation is a single nested list comprehension using the str-conversion idiom for digit extraction:
[int(d) for num in nums for d in str(num)]
This is a two-level flatten: the outer loop iterates over each number, str(num) breaks it into character digits, and int(d) converts each character back to an integer. The left-to-right ordering of for clauses preserves the original digit order across all numbers.
This pattern — int(d) for d in str(n) — is the standard Python idiom for digit decomposition without arithmetic (% 10, // 10), and it appears across many solutions in this repo (e.g., add-digits, alternating-digit-sum, count-the-digits-that-divide-a-number).
Imports: None — pure Python, no standard library or third-party dependencies.
Imported by: The "Imported By" list in the prompt is misleading — it lists ~400+ test files across the entire repo. These are almost certainly importing from their *own* solution.py (via a shared test harness or relative import), not from this specific file. The actual consumer is separate-the-digits-in-an-array/test_solution.py.
1. Caller passes nums, e.g. [13, 25].
2. Outer generator iterates: num = 13, then num = 25.
3. str(13) yields "13" → inner loop produces int("1")=1, int("3")=3.
4. str(25) yields "25" → inner loop produces int("2")=2, int("5")=5.
5. Returns [1, 3, 2, 5].
The entire transformation is eager (list comprehension, not generator), so the full result is materialized in one pass.
nums contains positive integers (per the LeetCode problem constraints: 1 <= nums[i] <= 10^5). Negative numbers would produce "-" as a character, causing int("-") to raise ValueError.7 produces [7] — str(7) is "7", int("7") is 7.None. The function trusts its input matches the LeetCode constraints. No validation, no try/except. A 0 in the input works fine (str(0) → "0" → [0]), but negative numbers or non-integers would blow up at int(d) or produce wrong results.
separate-the-digits-in-an-array/test_solution.py — See the test cases and edge cases covered for this solutionseparate-the-digits-in-an-array/review.md — Code review notes that may discuss alternative approaches (arithmetic vs. string conversion)alternating-digit-sum/solution.py:separate_digits — Another digit-decomposition problem; compare whether it uses the same str-conversion idiom or arithmeticdigit-extraction-patterns — Survey which solutions use str(n) vs. n % 10 / n // 10 for digit work and whether there's a performance difference at LeetCode scalestr-conversion-for-digits — separate_digits uses str(num) + int(d) for digit extraction rather than modular arithmetic, which preserves left-to-right digit order without reversalno-input-validation — The function performs no validation on nums; negative integers would raise ValueError on the "-" charactersingle-pass-eager — The list comprehension materializes the full result in a single pass with O(total_digits) time and spacezero-dependency — The module has no imports; it uses only Python builtins (str, int, list comprehension)