* Update summary.md
@Richard-Zhao93 的回答非常好,同时觉得"如果在开头能了解到这部分s,对初学者来说,可能更容易接受这些陌生的概念。"的建议很有道理
* Update QA of time_complexity
* Enhance explanation of hash tables vs arrays
Clarify the advantages of hash tables over arrays for key-value mapping.
* Update summary.md
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Co-authored-by: Yudong Jin <krahets@163.com>