Looking for outer contour lines

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C#

Hello everyone,
Looking for some ideas about how to find contour lines from intersecting lines set.

Line set for example looks like this :

+----+----------+
|    |     2    |
| 1  +----------+
|    |
+----+

where
|
|
|
is one line, and
|
|
is also one line. From this picture I have 6 lines which are not in any order.
Box 1 and box 2, each made up of 4 line segments, share a common side.

In result what i would like to get is such contour, made up of 6 line segments:

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

Maybe someone has some ideas of how to achieve this?