File: most-frequent-even-element/solution.py

Date: 2026-06-06

Time: 18:07

Purpose

This file is a self-contained LeetCode solution for problem 2404: Most Frequent Even Element. It owns both the algorithm implementation and its test suite in a single module — the standard pattern across this repository's ~500+ problem directories.

Key Components

mostfrequenteven(nums: list[int]) -> int

The sole public function. Contract:

• Input: A list of non-negative integers.
• Output: The most frequent even element. If there's a tie in frequency, returns the smallest among the tied values. Returns -1 if no even elements exist.

TestMostFrequentEven (unittest.TestCase)

Nine test cases covering:

• LeetCode's three provided examples (basic, dominant element, all-odd)
• Edge cases: single even, single odd, zero, all-same
• Tie-breaking: two-way tie, four-way tie — both verify the "smallest wins" rule

Patterns

Filter-count-select via Counter: The solution uses a generator expression inside Counter() to simultaneously filter (even only) and count in one pass. This is a common idiom in this repo for frequency problems — it avoids a separate filtering step.

Composite sort key for tie-breaking: min(counts, key=lambda x: (-counts[x], x)) is a classic Python trick. By negating the count, min finds the *maximum* frequency first, then among ties picks the smallest value. This collapses what would otherwise be a two-pass operation (find max freq, then filter and min) into a single min call.

Inline tests: Tests live in the same file as the solution rather than in a separate testsolution.py. The testsolution.py file exists in the directory but likely imports from this module.

Dependencies

Imports:

• collections.Counter — frequency counting
• unittest — test framework

Imported by: The testsolution.py files listed in the "Imported By" section (~400+ files) suggest a shared test harness or import pattern across the repo. The function is likely imported by most-frequent-even-element/testsolution.py directly, while the other test files listed are probably a repo-wide cross-reference artifact (they don't actually import *this* file — they follow the same structural pattern).

Flow

1. Build a Counter over only even elements: Counter(x for x in nums if x % 2 == 0)

2. If the counter is empty (no even elements), return -1

3. Find the element with the highest count, breaking ties by smallest value, using min with a composite key (-count, value)

The entire algorithm is O(n) time and O(n) space where n is the length of nums.

Invariants

• The function never raises — it returns -1 as the sentinel for "no even elements" rather than raising ValueError or returning None.
• Tie-breaking is deterministic: smallest value always wins among equally frequent elements.
• Zero is treated as even (which is mathematically correct and tested explicitly).

Error Handling

There is none, by design. The function assumes valid input per the LeetCode contract (non-empty list of non-negative integers). No input validation, no try/except. Empty counter is the only "error" path and it returns the sentinel -1.

Topics to Explore

• [file] most-frequent-even-element/test_solution.py — See how the external test file imports and exercises this solution, and whether it adds cases beyond the inline tests
• [file] most-frequent-even-element/review.md — Code review notes may document alternative approaches or performance observations
• [function] most-frequent-number-following-key-in-an-array/solution.py:mostFrequentFollowingKey — A related frequency-counting problem that likely uses a similar Counter pattern with different filtering logic
• [general] counter-with-tiebreak-pattern — Several solutions in this repo (largest-unique-number, count-common-words) use the same Counter + composite-key-min/max idiom — worth comparing implementations
• [file] run_tests.py — The repo-level test runner that orchestrates test execution across all problem directories

Beliefs

• most-frequent-even-returns-negative-one-sentinel — mostfrequenteven returns -1 (not None or an exception) when the input contains no even numbers
• tiebreak-selects-smallest-value — When multiple even elements share the highest frequency, the function returns the numerically smallest one
• single-pass-filter-and-count — Even filtering and frequency counting happen in a single pass via a generator inside Counter(), not as separate list comprehension + count steps
• zero-is-even — The function correctly classifies 0 as an even number, verified by an explicit test case