科技公司 SWE 在线评估：哈希表与集合题型精讲
SWE Online Assessment: Hash Map and Set Patterns
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摘要 Summary
哈希表和集合能换来平均 O(1) 的查找，而在线评估里相当多的题，本质上就是在考你「用什么当 key」。这篇文章讲清补数法、频次表、按签名分组，以及用一个 seen 集合在一遍扫描里发现重复。
Hash maps and sets buy you O(1) average lookups, and a surprising number of assessment problems are really about choosing the right key. This guide covers the complement trick, frequency maps, grouping by a signature, and using a seen-set to detect duplicates in one pass.
哈希表或集合能提供平均 O(1) 的插入和查找，而在线评估里很大一部分题，真正考的其实是一个决定：key 该选什么？一旦选对了 key——某个值、某个计数、或某种归一化后的签名——剩下的解法往往就是一遍扫描。所以这一类题虽然占比不大，却很值得练熟。
这篇文章整理自公开、广为人知的算法题型，而不是某家公司的真实原题。把它当成一张练习地图：先记住每种题的识别信号，每个模式手里留一份干净的参考解法，剩下的时间用来把题读懂，读到能一眼选对套路。
值得记住的几个模式| The Patterns Worth Knowing
补数法：一边扫描，一边查「我还差的那个值」是不是已经见过——这是两数之和这一类题的核心。
频次表：统计出现次数，再回答关于出现最多、最少或恰好 k 次的问题。
按签名分组：把每个元素映射到一个规范化的 key（比如排序后的字母），让等价的元素归到一起。
seen 集合去重：维护一个「已见过」的集合，一遍扫描就能发现第一个重复，或过滤掉重复项。
怎么一眼认出是哪一种| How to Recognise Each One
「找到两个加起来等于目标的东西」，几乎总是补数法。
「分组」「变位词」「在某种规则下等价的元素」，指向按签名分组。
「第一个重复」「是否含重复」「唯一」，就是 seen 集合。
只要涉及「出现最多」「前 k 个」，都从频次表起步。
一个例子：一遍扫描的两数之和| A Worked Example: Two Sum in One Pass
暴力做法要检查每一对，是 O(n^2)。哈希表版本只扫一遍：对每个数，问它的补数（target 减去这个数）是不是已经出现过。出现过就得到答案；没有就把当前数记下来，继续走。
def two_sum(nums, target):
seen = {} # value -> index
for i, value in enumerate(nums):
need = target - value
if need in seen: # complement already passed by
return [seen[need], i]
seen[value] = i # record only after checking
return []顺序很关键：先查补数，再插入当前值，否则单个元素可能会和自己配对。正是这种一行的顺序细节，决定了你的解法是全部通过，还是在某个隐蔽用例上翻车，所以要讲清楚为什么插入要放在最后。
最容易失分的几个点| Common Mistakes That Cost Marks
在查补数之前就插入当前值，导致元素和自己配对。
本该用集合（O(1)）却用了列表的 in 判断（O(n)），悄悄把解法退回 O(n^2)。
选了可变或不稳定的 key，比如用列表，而不是元组或归一化后的字符串。
忘了哈希是用内存换速度——在内存受限时，要主动说明这多出来的 O(n) 空间。
一个够用的练习计划| A Focused Practice Plan
每道题先定 key，并在写别的之前先用注释写下来。
把补数法和按签名分组各练到形成条件反射。
每次想用哈希表时，先问一句：普通数组下标或集合是不是更简单。
把时间和空间成本说出来，因为「内存换速度」是很常见的追问。
常见问题 FAQ
哈希表一定是最快的选择吗？
Is a hash map always the fastest choice?
不一定。当 key 是较小的整数时，用值直接做下标的普通数组更快、更省内存。key 空间很大或非数值时，才用哈希表。
为什么两数之和遇到重复数字会出错？
Why does two sum fail when there are duplicate numbers?
通常是因为你在查补数之前就插入了当前值。先查、后插，重复数字就能正确处理。
分组类问题该用什么当 key？
What key should I use for grouping problems?
用一个对所有等价元素都相同的规范形式——变位词用排序后的字符，或者一个计数元组。key 必须是不可变、可哈希的。
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