TreeSet 是 Java 中的一個(gè)集合類，它通過紅黑樹實(shí)現(xiàn)元素自動(dòng)排序和去重。其內(nèi)部基于自平衡二叉搜索樹（紅黑樹），支持 O(log n) 時(shí)間復(fù)雜度的插入、刪除和查找操作；添加元素時(shí)根據(jù)自然順序或自定義比較器自動(dòng)排序；適用于需要元素始終有序、快速檢索極值及避免重復(fù)的場(chǎng)景，如排行榜或優(yōu)先隊(duì)列；基本操作包括 add、first、last、floor、ceiling 等；局限包括性能開銷大于 HashSet、非線程安全、不支持 null 值（除非自定義比較器處理）；且僅適用于 Java 語言。

A TreeSet is a data structure in Java that stores elements in a sorted and ascending order. It’s part of the Java Collections Framework and implements the NavigableSet interface. One key thing to know: it doesn’t allow duplicate values, and it keeps everything automatically sorted — no extra work needed from you.

How Does TreeSet Work Internally?

TreeSet uses a self-balancing binary search tree under the hood — specifically, a Red-Black Tree. This means insertions, deletions, and lookups are all relatively fast, usually taking O(log n) time.

• When you add an element, it finds the right place based on the natural ordering (or a custom comparator if provided).
• The sorting happens automatically — you don't have to call sort() or anything like that.
• Since it's a balanced tree, performance remains stable even as the number of items grows.

One important note: if you're storing custom objects (like a Person class), you must define how they should be ordered — either by implementing Comparable, or by passing a Comparator when creating the TreeSet.

When Should You Use TreeSet?

Use a TreeSet when:

• ? You need elements to stay sorted at all times
• ? You want fast retrieval of elements (e.g., finding the smallest or largest value)
• ? You want to avoid duplicates without manually checking

It’s especially handy for things like leaderboards, priority queues, or maintaining a running list of unique values that you frequently query in order.

If you only need sorting once in a while, or don’t care about order most of the time, consider using a regular HashSet and sorting when needed — it’ll save some overhead.

Basic Operations with TreeSet

Here are the most common actions you'll perform:

• Adding elements: Just use .add(value) — duplicates are ignored silently.
• Getting the first or last: .first() and .last() give you the smallest and largest elements.
• Finding closest matches: Methods like .floor(), .ceiling(), .lower(), and .higher() help find nearby values.
• Subsets and navigation: You can get ranges using .subSet(), .headSet(), or .tailSet().

For example, if you have a TreeSet of integers:

TreeSet<Integer> numbers = new TreeSet<>();
numbers.add(5);
numbers.add(2);
numbers.add(8);
System.out.println(numbers); // Output: [2, 5, 8]

And retrieving the next higher value after 5 would be:

System.out.println(numbers.higher(5)); // Output: 8

Limitations to Be Aware Of

TreeSet isn’t perfect for every situation:

• ? Slower than HashSet for simple lookups and insertions due to sorting overhead
• ? Not thread-safe — you’ll need external synchronization if used in concurrent environments
• ? Null values aren’t allowed unless you’re using a custom comparator that defines how nulls should be handled

Also, keep in mind that TreeSet is specific to Java. If you're working in another language, similar functionality might come from structures like std::set in C++ or SortedSet in Python via third-party modules.

基本上就這些。

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