Google Interview Question






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The question obviously points to a Graph solution.
Starting from the root C, we should implement BFS using a queue and a "visited" matrix. We can also keep a "parent" matrix to remember the different paths.
To find the turning off order, we can do the topological sort of the graph.

- Lorenzo April 13, 2019 | Flag Reply
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topological sort is useless here since its not DAG, its undirected graph.

- Anonymous April 14, 2019 | Flag
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Given a computer at (row, col) put it into the processing queue.
Then, take first element to process and:
1. Skip if already processed.
2. If it is a computer, print it and put its row (row, 0) and column (0, col) for processing.
3. If it is a row, find all computers in that row and put for processing.
4. If it is a column, find all computers in that column and put for processing.
Assumption: indices start from 1, while 0 denotes the entire row/column.

#include <algorithm>
#include <iostream>
#include <list>
#include <map>
#include <set>
#include <vector>

class Node
{
public:
	int row;
	int col;
	class CompareByRow
	{
	public:
		bool operator()(const Node* x, const Node* y) { return x->row < y->row; }
	};
	class CompareByCol
	{
	public:
		bool operator()(const Node* x, const Node* y) { return x->col < y->col; }
	};
	bool operator<(const Node& o) const { return row < o.row || row == o.row && col < o.col; }
};

int main()
{
	std::vector<Node> computers = { {1, 1}, {2, 2}, {4, 4}, {4, 1} };
	std::vector<const Node*> comp_by_row;
	std::vector<const Node*> comp_by_col;
	for (const Node& n : computers)
	{
		comp_by_row.push_back(&n);
		comp_by_col.push_back(&n);
	}
	std::sort(comp_by_row.begin(), comp_by_row.end(), Node::CompareByRow());
	std::sort(comp_by_col.begin(), comp_by_col.end(), Node::CompareByCol());

	std::list<Node> processing_queue;
	std::set<Node> processed_nodes;
	processing_queue.push_back(computers[0]);
	while (!processing_queue.empty())
	{
		Node& n = processing_queue.front();
		if (processed_nodes.insert(n).second)
		{
			if (n.row != 0 && n.col != 0)
			{
				std::cout << "Node (" << n.row << ", " << n.col << ") added." << std::endl;
				processing_queue.push_back({ n.row, 0 });
				processing_queue.push_back({ 0, n.col });
				processed_nodes.insert(n);
			}
			else
			{
				if (n.row == 0)
				{
					auto b = std::lower_bound(comp_by_col.begin(), comp_by_col.end(), &n, Node::CompareByCol());
					auto e = std::upper_bound(comp_by_col.begin(), comp_by_col.end(), &n, Node::CompareByCol());
					while (b != e)
					{
						processing_queue.push_back(**b);
						b++;
					}
				}
				if (n.col == 0)
				{
					auto b = std::lower_bound(comp_by_row.begin(), comp_by_row.end(), &n, Node::CompareByRow());
					auto e = std::upper_bound(comp_by_row.begin(), comp_by_row.end(), &n, Node::CompareByRow());
					while (b != e)
					{
						processing_queue.push_back(**b);
						b++;
					}
				}
			}
		}
		else
		{
			std::cout << "Node (" << n.row << ", " << n.col << ") skipped (already processed)." << std::endl;
		}
		processing_queue.pop_front();
	}
	return 0;
}

- hb April 15, 2019 | Flag Reply


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