> For the complete documentation index, see [llms.txt](https://mm.ricky.moe/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mm.ricky.moe/algorithm/algorithm-and-data-structure/shu-ju-jie-gou/ji-he.md).

# 集合

集合是由一组无序且唯一的元素组成的数据结构，由于不会重复，所以集合中键就是值。

```javascript
class MySet extends Set {
  constructor (...args) {
    super(...args)
  }
  union (otherSet) { // 并集
    return new MySet([...this, ...otherSet])
  }
  intersection (otherSet) { // 交集
    // return new Set([...this].filter(x => otherSet.has(x))) // 标记 aa
    let newSet = new MySet()
    for (let a of this) {
      if (otherSet.has(a)) {
        newSet.add(a)
      }
    }
    return newSet
  }
  difference (otherSet) { // 差集
    // return new Set([...this].filter(x => !otherSet.has(x))) // 标记 bb
    let newSet = new MySet()
    for (let x of this) {
      if (!otherSet.has(x)) {
        newSet.add(x)
      }
    }
    return newSet
  }
  isSubOf (otherSet) { // 子集
    if (this.size > otherSet.size) {
      return false
    } else {
      // return [...this].every(item => otherSet.has(item)) // 标记 cc
      for (let x of this) {
        if (!otherSet.has(item)) {
          return false
        }
      }
      return true
    }
  }
}
var a = new MySet([1, 2, 3, 4])
var b = new MySet([3, 4, 5, 6])
var c = new MySet([1,2])
a.intersection(b) // Set(2) {3, 4}
a.difference(b) // Set(2) {1, 2}
a.union(b) // Set(6) {1, 2, 3, 4, 5, 6}
c.isSubOf(a) // true
c.isSubOf(b) // false
```

上面代码中标记的“aa bb cc”三处一行代码就可以实现其功能，但是其时间复杂度为 O(n²)，相比之下其下面的代码时间复杂度为 O(n)。因为 `[...this]` 先遍历集合中所有元素转为数组，然后用`every`遍历数组中的元素。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://mm.ricky.moe/algorithm/algorithm-and-data-structure/shu-ju-jie-gou/ji-he.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.