Custom Comparator Drills

01 · Question

Use the language's built-in sort for each task:

Sort strings in descending lexicographic order, ignoring case.
Sort intervals by their end value.
Sort cards by value, breaking ties with clubs < hearts < spades < diamonds.
Sort cards in new-deck order: hearts < clubs < diamonds < spades, then by value.

02 · Analysis

Describe the ordering key before writing syntax:

Transform case when comparison should ignore it.
Select the exact field that defines the order.
Use a numeric rank map for categories with a custom order.
Use multiple key fields to make tie-breakers explicit.

The built-in comparison sort takes $O(n log n)$ comparisons. Include any non-constant key or comparison work in the final complexity.

03 · Solution

Built-in sort patterns

1. Case-insensitive descending order

1def case_insensitive_sort(strings):
2    return sorted(strings, key=str.lower, reverse=True)

2. Sort intervals by end

1def sort_by_interval_end(intervals):
2    return sorted(intervals, key=lambda interval: interval[1])

3. Sort cards by value, then suit

1def sort_cards(deck):
2    suit_rank = {
3        "clubs": 0,
4        "hearts": 1,
5        "spades": 2,
6        "diamonds": 3,
7    }
8    return sorted(
9        deck,
10        key=lambda card: (card.value, suit_rank[card.suit]),
11    )

4. Sort cards in new-deck order

1def new_deck_order(deck):
2    suit_rank = {
3        "hearts": 0,
4        "clubs": 1,
5        "diamonds": 2,
6        "spades": 3,
7    }
8    return sorted(
9        deck,
10        key=lambda card: (suit_rank[card.suit], card.value),
11    )

Kth Largest Element in an Array

Stable Sort by Value