Falling Dominoes

The title alone cannot determine whether dominoes are in a line, graph, or grid, whether they fall instantly or over time, or how simultaneous opposing pushes behave. Ask those questions first. In the common line version, model each initial push as a timed event and use a queue or two directional sweeps to compute when force reaches each standing domino. A domino hit from one side falls that way; equal-time hits from opposite sides remain upright if that is the stated physics. The invariant is that once the earliest applicable event for a position is resolved, later events cannot reverse a completed fall unless the rules explicitly permit it. A simpler two-sweep solution records nearest rightward and leftward force distances, then compares them. Test no initial pushes, all falling one way, equal-time collision, boundaries, and consecutive already-fallen dominoes. With fixed local propagation, time and space are O(n). Do not claim a particular collision rule without the statement.