Editorial for DMOPC '17 Contest 5 P6 - Bridges


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Author: r3mark

There is no intended solution for the first subtask.

For the second subtask, specific information can be gained by asking an order, then swapping two consecutive elements of this order and asking that. In particular, consider the queries n-1 n-2 ... 4 3 2 1 and n-1 n-2 ... 4 3 1 2. Let the answers to these two queries by A_1 and A_2 respectively. Let bridge X be the leftmost bridge which has not yet been built when bridge 1 is built in the advisor's order. It can be shown that if A_1 \ge A_2, then X must be 2, otherwise, v_2+v_3+\ldots + v_X=\frac{A_2-A_1+2v_1v_2}{2v_1} (it is possible for X to be n, in which case, bridge 1 is the last bridge to be built).

So now, we know that no bridge with index between 1 and X can be built after bridge 1 and before bridge X. We now repeat this solution for the bridges from 2 to X-1 and the bridges from X+1 to N-1. We also have to update the v array and merge it appropriately. Clearly this solution requires less than 2N queries. The time complexity is unimportant, it is at the very most \mathcal O(QN).


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