Find Players With Zero or One Losses

Find Players With Zero or One Losses

What is this problem about?

The Find Players With Zero or One Losses interview question is a tournament tracking problem. You are given a list of matches where each match is a pair [winner, loser]. You need to return two lists:

1. Players who have not lost any matches.
2. Players who have lost exactly one match. The results should be sorted by player ID.

Why is this asked in interviews?

Amazon and Palantir ask the Find Players coding problem to test your proficiency with Hash Tables and data aggregation. It evaluations whether you can distinguish between different counts (total wins don't matter, only the count of losses) and how you handle players who only appear as winners (and thus have 0 losses).

Algorithmic pattern used

This problem uses the Frequency Counting and Set Union patterns.

1. Loss Counter: Use a hash map to count how many times each player appears as a loser.
2. All Players: Use a hash set to collect every player ID mentioned in the matches (winners and losers).
3. Classification:
   • If a player is in the "All Players" set but NOT in the "Loss Counter," they have 0 losses.
   • If a player has a count of exactly 1 in the "Loss Counter," they have 1 loss.
4. Sorting: Sort both resulting lists.

Example explanation

Matches: [[1,3], [2,3], [3,6], [5,6]]

• Winners: {1, 2, 3, 5}
• Losers: {3 (once), 3 (twice), 6 (once), 6 (twice)} -> Map: {3: 2, 6: 2}.
• Check all:
  • 1: No losses. (List 1)
  • 2: No losses. (List 1)
  • 3: 2 losses. (Ignore)
  • 5: No losses. (List 1)
  • 6: 2 losses. (Ignore) Result: [[1, 2, 5], []].

Common mistakes candidates make

• Ignoring Winners: Forgetting to track players who only win and never lose. They must be in the "zero losses" list.
• Incorrect Sorting: Returning the lists in the order players were found instead of sorted order.
• Duplicate Players: Not using a Set or Map correctly, leading to players being listed multiple times.

Interview preparation tip

When a problem asks for information about "players" based on "matches," always identify the distinct roles (Winner vs Loser). In this Hash Table interview pattern, the number of wins is irrelevant—only the existence of a player and their loss count matter.