Trees

Trees are the data structure where recursion becomes natural. Most tree problems follow DFS, going deep before backtracking, or BFS, processing level by level. DFS is usually recursive and suits path-based problems, while BFS uses a queue and suits level-based or shortest-path questions. A key habit is separating what a function returns from what it records as a side effect, since many problems need a global answer tracked alongside local recursive decisions. Segment trees and Fenwick trees extend these ideas to range queries over a mutable array, though they show up more in competitive programming than standard interviews.