Graph BFS

Strip a tree of its no-cycles guarantee, or a grid of its coordinate math, and what's left is a general graph. The engine never changes: pull from a queue, process the node, push its neighbors. Across trees, grids, and general graphs only two things move: how you find a node's neighbors, and whether you have to guard against revisiting. Trees have no cycles, so no visited set. A grid is a graph too, with each cell's neighbors being the adjacent cells. General graphs spell their edges out and can loop back on themselves, so drop the visited set and BFS runs until the queue eats all your memory.