Date: 2026-06-06
Time: 18:56
This file is a self-contained solution to LeetCode 2103: Rings and Rods. It owns both the algorithm implementation and its test suite in a single module — the standard pattern across this repository's ~500+ problem directories.
The problem: given a string encoding color-rod pairs (e.g., "B0R0G0"), count how many rods have all three colors (Red, Green, Blue) placed on them.
Solution.countPoints(rings: str) -> intThe core algorithm. Takes a string where each pair of characters represents a color (R/G/B) followed by a rod number (0-9), and returns the count of rods that collected all three colors.
TestSolutionEight test cases covering: the three LeetCode examples, full coverage (all 10 rods with all colors), single-rod edge cases, duplicate colors on one rod, two-color insufficiency, and multi-rod qualifying scenarios.
Set accumulation with defaultdict(set) — Each rod maps to a set of colors. Using a set automatically deduplicates repeated colors on the same rod, which is exactly what the problem requires. The final check len(colors) == 3 works because there are exactly three possible colors.
Stride-2 iteration — range(0, len(rings), 2) walks the string in pairs. This avoids explicit parsing or regex — the input format guarantees alternating color-char/digit-char pairs, so fixed-stride indexing is the cleanest approach.
Counting via generator expression — sum(1 for ... if ...) is the idiomatic Python pattern for conditional counting without materializing an intermediate list.
Imports: collections.defaultdict (core data structure), unittest (test harness).
Imported by: The test_solution.py in this same directory, plus the "Imported By" list in the prompt is misleading — that list appears to be an artifact of the repository tooling referencing a shared test runner or import pattern across all problem directories, not actual imports of this specific solution.
1. Initialize rods as a defaultdict(set) — keys are rod identifiers (single-char strings '0'–'9'), values are sets of color characters.
2. Walk rings two characters at a time: extract the color at position i and the rod at position i+1.
3. Add each color to its rod's set (duplicates are absorbed by the set).
4. Count and return the number of rods whose color set has exactly 3 elements.
For input "B0R0G0R9": after the loop, rods = {'0': {'B', 'R', 'G'}, '9': {'R'}}. Rod '0' has 3 colors → counts. Rod '9' has 1 → doesn't. Returns 1.
'0'–'9'), so there are at most 10 rods.3 in len(colors) == 3 is tied to exactly three possible colors (R, G, B). If the problem added a fourth color, this would need updating.None. The solution trusts the problem's input constraints. An odd-length string would silently produce a wrong answer (the last character would be treated as a color with no rod). An empty string returns 0 correctly since rods stays empty.
rings-and-rods/test_solution.py — Separate test file that likely imports this Solution class; compare with the inline tests hererings-and-rods/review.md — Code review notes that may document alternative approaches (bitmask, Counter-based)set-accumulation-pattern — Many problems in this repo use defaultdict(set) for group membership tracking; compare with check-if-the-sentence-is-pangram and count-the-number-of-consistent-stringsrings-and-rods/solution.py:countPoints — Consider the bitmask alternative: 3 bits per rod (R=1, G=2, B=4), check for == 7, avoids set overhead entirelyrun_tests.py — The repo-wide test runner that exercises all solution files including this onerings-input-stride-2 — countPoints assumes the input string has exactly 2 characters per ring placement (color + rod digit) and does not validate length parityrod-completion-threshold-hardcoded — The completion check uses the literal 3, coupling it to the RGB color set; a fourth color would require changing this constantdefaultdict-set-dedup — Duplicate color placements on the same rod are harmlessly absorbed by the set, making the solution idempotent per rod-color pairrod-keys-are-strings — Rod identifiers are stored as single-character strings (not integers), since they're extracted directly from the input string without conversion