亚马逊面试题：从网站日志中统计最常访问的三页面序列模式

Amazon has web server logs from multiple hosts.
Each log entry consists of:

• a Session ID
• a Web Page ID

Example (conceptual):

session_id, web_page_id
0001, product_1
0001, cart
0002, product_2
0003, home_page
0005, help
0001, checkout
0002, cart
0001, help
0001, product_1
0002, checkout
0005, checkout
0002, help

We want to find the 3-page pattern that is visited most frequently across all sessions.

In the example, the sequence <cart, checkout, help> appears twice, so it is the most frequent 3-page pattern.

Given logs in the form:

logs = [
  session:0001, web:product_1
  session:0001, web:cart
  session:0002, web:product_2
  session:0003, web:home_page
  session:0005, web:help
  session:0001, web:checkout
  session:0002, web:cart
  session:0001, web:help
  session:0001, web:product_1
  session:0002, web:checkout
  session:0005, web:checkout
  session:0002, web:help
]

Return the 3-page sequence that is most frequently visited, counting only in-order sequences within the same session.

先按 session_id 分组，并按时间顺序排列每个会话的页面，然后在每个会话里用大小为 3 的 滑动窗口 枚举所有三页面序列，用哈希表统计出现次数，最后返回出现次数最多的那一个三页面模式。

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