3 entries — one card per concept, for looking something up rather than learning it. Each links back to the full explanation.
def solve(input: T) -> R | None: """Exact contract, including the None/edge case."""
A precise restatement fixes input/output types and every edge case (ties, no match, empty input) before coding starts.
def two_sum_indices(prices: list[int], target: int) -> tuple[int, int] | None: ...
O(1) < O(log n) < O(n) < O(n log n) < O(n²) < O(2ⁿ) < O(n!)
The complexity classes worth recognizing on sight, fastest to slowest.
n=20: 1, ~4, 20, ~86, 400, 1,048,576, 2,432,902,008,176,640,000
x in list_obj # O(n) | x in set_obj / dict_obj # O(1) average
The same `in` syntax costs very different amounts depending on the container.
existing = set(existing_items); [c for c in candidates if c not in existing]
