> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_7_data_structure/stack--queue.md).

# Stack & Queue

## Queue

Operations:

* push: `O(1)`
* pop: `O(1)`
* Top: `O(1)`

  **Always used for BFS**

## Stack

Operations:

* push: `O(1)`
* pop: `O(1)`
* Top: `O(1)`