System Design — Game Leaderboard

Scores go up and down, and the product needs "top K" and "my rank" at any time.

solve(args, ops) builds new Leaderboard() and replays ops:

• add(player, delta) — add delta (may be negative) to the player's score; an unknown

player starts at 0. Returns the player's new score.

• top(k) — return the ids of the k best players, best first. Fewer players than k is fine.
• rank(player) — the 1-based position of the player, or -1 for an unknown player.
• reset(player) — remove the player from the board. Returns nothing.

Ordering is pinned: higher score first; on equal scores the smaller player id (plain
string comparison) comes first. Players with equal scores therefore do *not* share a rank.

A sort on every query is acceptable here. In production you would keep a sorted structure
(balanced tree, skip list or Redis sorted set) so top and rank are O(log n) — be ready to
say so.

Example

add("a",10) add("b",20) add("c",20)  top(2) → ["b","c"]   rank("a") → 3