Micro-Drill #221 — BFS vs DFS: when to use which

BFS vs DFS — DECISION GUIDE
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USE BFS when:
  □ Shortest path in unweighted graph
  □ Level-by-level processing needed
  □ Closest node satisfying condition
  □ Minimum steps / moves / transforms
  Examples: word ladder, shortest grid path, rotten oranges

USE DFS when:
  □ Exhaustive search / enumerate all paths
  □ Cycle detection, connected components
  □ Topological sort
  □ Tree traversals (inherently DFS)
  □ Backtracking problems
  Examples: number of islands, path exists, clone graph

COMPLEXITY: both O(V + E)

SPACE:
  BFS: O(width) — queue stores entire level
  DFS: O(height/depth) — stack stores path

GRID PROBLEMS:
  "Shortest path"  → BFS (always)
  "Can you reach?"  → DFS (simpler) or BFS
  "Count regions"   → DFS or BFS (both fine)
  "All paths"       → DFS with backtracking