补数查找（哈希表） 模式

识别信号

• Do you recognize that Two Sum only needs a complement existence check, not comparison against every later element?
• Can you explain why checking the hash map before inserting the current value avoids reusing the same index?

解题流程

1. 1. 明确窗口/状态定义
2. 2. 更新状态并维护约束
3. 3. 用边界样例验证

常见失分点

• Inserting the current value before checking its complement can incorrectly pair an element with itself when the target is twice that value.