> For the complete documentation index, see [llms.txt](https://jaywin.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jaywin.gitbook.io/leetcode/topics/dfs-bfs.md).

# 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)


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