LeetCode 787 Cheapest Flights Within K Stops - Temporal Separation

If you update $dist$ in place, you might use a newly-updated value in the same iteration. This allows paths longer than the iteration count. Example: Edge $A \to B$ updates dist[B], then edge $B \to C$ uses the new dist[B] in the same round. That allows a $2$-edge path in one iteration, violating the constraint.

Using prev ensures each iteration extends paths by exactly one edge. You read from prev and write to $dist$, keeping layers separate. This is temporal separation: each iteration represents one time step.