Recursion

Find max depth of DOM tree

Collect unique tags in DOM tree

Count paths with target sum (not necessarily root-to-leaf)

Fill connected pixels in an image with a replacement color

Deep clone an undirected graph given a start node

Determine whether a path exists between two vertices

Count connected components in an adjacency matrix

Support addWord and search with wildcard dot matching

Implement a stream checker that detects any suffix matching inserted words

Find all words in a board using trie-guided DFS

Implement buildDict + search allowing one character mismatch

Find the longest word that can be built one character at a time

Implement merge sort using divide-and-conquer

Implement quicksort with partition — the most important sort for interviews

Find the kth largest element using QuickSelect (partition)