## IOI '20 P2 - Connecting Supertrees

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Points: 15 (partial)
Time limit: 2.0s
Memory limit: 512M

Problem type
Allowed languages
C++

Gardens by the Bay is a large nature park in Singapore. In the park there are towers, known as supertrees. These towers are labelled to . We would like to construct a set of zero or more bridges. Each bridge connects a pair of distinct towers and may be traversed in either direction. No two bridges should connect the same pair of towers.

A path from tower to tower is a sequence of one or more towers such that:

• the first element of the sequence is ,
• the last element of the sequence is ,
• all elements of the sequence are distinct, and
• each two consecutive elements (towers) in the sequence are connected by a bridge.

Note that by definition there is exactly one path from a tower to itself and the number of different paths from tower to tower is the same as the number of different paths from tower to tower .

The lead architect in charge of the design wishes for the bridges to be built such that for all there are exactly different paths from tower to tower , where .

Construct a set of bridges that satisfy the architect's requirements, or determine that it is impossible.

#### Implementation details

You should implement the following procedure:

int construct(std::vector<std::vector<int>> p)

• : an array representing the architect's requirements.
• If a construction is possible, this procedure should make exactly one call to build (see below) to report the construction, following which it should return .
• Otherwise, the procedure should return without making any calls to build.
• This procedure is called exactly once.

The procedure build is defined as follows:

void build(std::vector<std::vector<int>> b)

• : an array, with if there is a bridge connecting tower and tower , or otherwise.
• Note that the array must satisfy for all and for all .

#### Examples

##### Example 1

Consider the following call:

construct({{1, 1, 2, 2}, {1, 1, 2, 2}, {2, 2, 1, 2}, {2, 2, 2, 1}})


This means that there should be exactly one path from tower to tower . For all other pairs of towers , such that , there should be exactly two paths from tower to tower . This can be achieved with bridges, connecting pairs of towers , , and .

To report this solution, the construct procedure should make the following call:

build({{0, 1, 0, 0}, {1, 0, 1, 1}, {0, 1, 0, 1}, {0, 1, 1, 0}}) It should then return .

In this case, there are multiple constructions that fit the requirements, all of which would be considered correct.

##### Example 2

Consider the following call:

construct({{1, 0}, {0, 1}})


This means that there should be no way to travel between the two towers. This can only be satisfied by having no bridges.

Therefore, the construct procedure should make the following call:

build({{0, 0}, {0, 0}})


After which, the construct procedure should return .

##### Example 3

Consider the following call:

construct({{1, 3}, {3, 1}})


This means that there should be exactly paths from tower to tower . This set of requirements cannot be satisfied. As such, the construct procedure should return without making any call to build.

#### Constraints

• •  for all •  for all •  for all 1. ( points)  for all 2. ( points) or  for all 3. ( points) or  for all 4. ( points)  for all and there is at least one construction satisfying the requirements.
5. ( points)  for all 6. ( points) No additional constraints.

• line : • line  : The output of the sample grader is in the following format:

• line : the return value of construct.

If the return value of construct is , the sample grader additionally prints:

• line  : 