File: largest-unique-number/solution.py

Date: 2026-06-06

Time: 17:18

Purpose

This file implements the solution to LeetCode problem 1133 - Largest Unique Number. It owns exactly one responsibility: given a list of integers, find the largest one that appears exactly once. If no such integer exists, return -1.

It's one of ~500+ problem solutions in the leetcode-implementations repo, following the standard per-problem directory layout (solution.py, test_solution.py, plan.md, review.md).

Key Components

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

The sole public function. Contract:

• Input: a list of integers (may contain duplicates, may be empty)
• Output: the largest integer with count == 1, or -1 if every element is duplicated or the list is empty
• Side effects: none — pure function

Patterns

Count-filter-reduce: a three-step idiom common across this repo's solutions:

1. Count — Counter(nums) builds a frequency map in O(n)

2. Filter — list comprehension keeps only elements where c == 1

3. Reduce — max() extracts the answer, with an empty-check guard

The ternary max(uniques) if uniques else -1 avoids a ValueError from calling max() on an empty sequence. This is the idiomatic Python approach — no sentinel values or try/except needed.

Dependencies

Imports: collections.Counter — the only dependency. No custom modules, no framework code.

Imported by: The largest-unique-number/test_solution.py file imports this function. The massive "Imported By" list in the prompt is misleading — those are test files for *other* problems that happen to share a common test harness or import pattern, not direct consumers of this function.

Flow

nums → Counter(nums) → {value: count} → filter(count == 1) → max() or -1

For input [5, 7, 3, 9, 4, 9, 8, 3, 1]:

1. Counter → {5:1, 7:1, 3:2, 9:2, 4:1, 8:1, 1:1}

2. Filter → [5, 7, 4, 8, 1]

3. max() → 8

Invariants

• The return value is always an int — either an element from nums or the literal -1
• -1 is returned if and only if uniques is empty (no element has count 1)
• Elements with count > 1 are never candidates, regardless of their magnitude

Error Handling

None explicit. The function relies on Counter accepting any iterable and max() being guarded by the truthiness check on uniques. An empty input list produces an empty Counter, an empty uniques list, and returns -1 — the correct answer per the problem spec.

Topics to Explore

• [file] largest-unique-number/test_solution.py — See what edge cases are covered (empty list, all duplicates, single element)
• [file] largest-unique-number/review.md — Read the code review for alternative approaches and complexity analysis
• [function] sum-of-unique-elements/solution.py:sumofunique_elements — Near-identical count-filter-reduce pattern but with sum() instead of max()
• [general] counter-based-solutions — Explore how many solutions in this repo use Counter as the core data structure
• [file] largest-unique-number/plan.md — Understand the planning/reasoning that led to this approach choice

Beliefs

• largest-unique-returns-minus-one-on-empty — largestuniquenumber([]) returns -1 because Counter({}) yields no items with count 1
• counter-filter-max-is-o-n — The solution runs in O(n) time and O(n) space where n is len(nums), since Counter is a single pass and the filter/max are bounded by the number of distinct values
• uniqueness-means-count-exactly-one — The filter c == 1 enforces that "unique" means appearing exactly once, not "distinct"; [3, 3] has no unique numbers
• no-exception-on-all-duplicates — The ternary guard if uniques prevents ValueError from max() on an empty list, making -1 the only error signal