Depth-first search (DFS) is a fundamental algorithm for working with trees, because it gives us a way to “visit” every node in the tree, once each.(1) That’s important, because visiting every node is a subproblem of many other problems on trees.
DFS explores the full depth of one subtree before moving onto the next one. If that’s the order we want to visit the nodes in, or the order doesn’t matter, then DFS is fine; otherwise, we need a different algorithm. We’ll see one in this post.