A14310 | Lost Tree

This is an interactive problem.

Little Dormi was faced with an awkward problem at the carnival: he has to guess the edges of an unweighted tree of $n$ nodes! The nodes of the tree are numbered from $1$ to $n$ .

The game master only allows him to ask one type of question:

- Little Dormi picks a node $r$ ( $1 \le r \le n$ ), and the game master will reply with an array $d_1, d_2, \ldots, d_n$ , where $d_i$ is the length of the shortest path from node $r$ to $i$ , for all $1 \le i \le n$ .

Additionally, to make the game unfair challenge Little Dormi the game master will allow at most $\lceil\frac{n}{2}\rceil$ questions, where $\lceil x \rceil$ denotes the smallest integer greater than or equal to $x$ .

Faced with the stomach-churning possibility of not being able to guess the tree, Little Dormi needs your help to devise a winning strategy!

Note that the game master creates the tree before the game starts, and does not change it during the game.

The first line of input contains the integer $n$ ( $2 \le n \le 2\,000$ ), the number of nodes in the tree.

You will then begin interaction.

When your program has found the tree, first output a line consisting of a single "!" followed by $n-1$ lines each with two space separated integers $a$ and $b$ , denoting an edge connecting nodes $a$ and $b$ ( $1 \le a, b \le n$ ). Once you are done, terminate your program normally immediately after flushing the output stream.

You may output the edges in any order and an edge $(a,b)$ is considered the same as an edge $(b,a)$ . Answering is not considered as a query.

Interaction

After taking input, you may make at most $\lceil\frac{n}{2}\rceil$ queries. Each query is made in the format "? r", where $r$ is an integer $1 \le r \le n$ that denotes the node you want to pick for that query.

You will then receive $n$ space separated integers $d_1, d_2, \ldots, d_n$ , where $d_i$ is the length of the shortest path from node $r$ to $i$ , followed by a newline.

After printing a query do not forget to output end of line and flush the output. Otherwise, you will get Idleness limit exceeded. To do this, use:

- fflush(stdout) or cout.flush() in C++;
- System.out.flush() in Java;
- flush(output) in Pascal;
- stdout.flush() in Python;
- see documentation for other languages.

If at any point you make an invalid query or try to make more than $\lceil \frac{n}{2} \rceil$ queries, the interaction will terminate immediately and you will receive a Wrong Answer verdict.

Hacks

To hack a solution, use the following format.

The first line contains the integer $n$ ( $2 \le n \le 2\,000$ ).

The next $n−1$ lines contain two integers $u$ and $v$ ( $1 \le u,v \le n$ ) denoting an edge between $u$ and $v$ ( $u \neq v$ ). These $n-1$ edges must form a tree.