You are developing an online game where 2 users can play against each other.
We want to create a service which will match users with similar ranks so that they have good experience in competing.
Design a system which can help to build this matching service.
这道题要求设计一个在线游戏的玩家匹配服务，核心目标是把 rank 相近的两个用户尽量配对，让对战体验更公平。解题时通常会从系统设计角度出发：如何接收玩家请求、按 rank 进行分桶或排序、快速找到可匹配对象，并处理等待队列、超时和并发问题。常见思路是使用优先队列、平衡树或按分数区间维护候选池，再结合分层匹配策略逐步放宽 rank 差距，从而在匹配速度和公平性之间取得平衡。
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