An old chestnut that has many forms goes something like this:
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In the diagram below, draw a continuous path that goes through each edge (defined as a line segment between two plus signs) exactly once.


  +---+---+---+
  |   |   |   |
  +---+-+-+---+
  |     |     |
  +-----+-----+

No cheating by going through corners, or by just coming up tangent to an edge without actually crossing it.

Find the line, or show why one does not exist.

Many variants exist with other diagrams, but this is one of the most popular.

Answer