Problem: Closing the Farm (USACO Gold)
Link: USACO 644
Task: Barns close one by one. Before each closing, is the remaining farm connected?
Process in reverse: start with no barns, add them in reverse closing order. Check connectivity after each addition. This is incremental, perfect for DSU. When you add a barn, union it with all its already-open neighbors. Track the component count to determine if the farm is connected.