Union-Find (Disjoint Set Union)

Union-Find (Disjoint Set Union) maintains a partition of elements into groups and answers 'are these two connected?' in near-constant amortized time via path compression and union by rank. Interviews test it because the naive alternative (re-running DFS or BFS per query) is too slow when merges repeat, and DSU is the right tool for dynamic connectivity, Kruskal's MST, and grouping problems where edges show up over time.