# DFS BFS

## Key points:

* DFS-stack\&recursion, BFS-queue
* If BFS works, never use DFS because of StackOverflow, important! like this one: [surrounded-regions](https://leetcode.com/problems/surrounded-regions/)
* DFS:
  * A and B is connected?
  * Connected component. (Union Find is faster because no need to build graph)
  * Search path
  * Has cycle
  * two color
  * Backtracking, all results
* BFS:
  * shortest path
  * connected component
  * topological sorting
  * level order traversal
  * Topological sorting
* Need to mark visited element:
  * if you need to get value, use `HashMap`
  * if just determine contain or not, use `HashSet` or `boolean[]`

![](https://kevhuang.com/content/images/2015/06/tree-traversal.gif)

## Templates

### BFS in a tree

* Queue -> q.offer(root) -> while -> levelNum = q.size() -> for loop
* if you don't need layer, `for loop` is not required.

```java
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Queue<TreeNode> q = new Queue<TreeNode>();

        if (root == null) return res;

        q.offer(root);
        while (!q.isEmpty()) {
            int levelNum = q.size();
            List<Integer> tmpList = new ArrayList<Integer>();
            for (int i = 0; i < levelNum; i++) {
                TreeNode curr = q.poll();
                tmpList.add(curr.val);
                if (curr.left != null) q.offer(curr.left);
                if (curr.right != null) q.offer(curr.right);
            }
            res.add(tmpList);
        }
        return res;
    }
}
```

### Permutation & Combination

![](https://pic3.zhimg.com/50/52a7a08bcfb9d133a8683ec26aa1924a_hd.jpg) ![](https://pic1.zhimg.com/50/8c70313549374daf4db1b8b436267f50_hd.jpg)


---

# Agent Instructions: 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:

```
GET https://jaywin.gitbook.io/leetcode/topics/dfs-bfs.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
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.