Added Solution for 2023 day 11

2023/day11_cosmic_expansion/challenge.txt

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+You continue following signs for "Hot Springs" and eventually come across an [observatory](https://en.wikipedia.org/wiki/Observatory). The Elf within turns out to be a researcher studying cosmic expansion using the giant telescope here.
+
+He doesn't know anything about the missing machine parts; he's only visiting for this research project. However, he confirms that the hot springs are the next-closest area likely to have people; he'll even take you straight there once he's done with today's observation analysis.
+
+Maybe you can help him with the analysis to speed things up?
+
+The researcher has collected a bunch of data and compiled the data into a single giant *image* (your puzzle input). The image includes *empty space* (`.`) and *galaxies* (`#`). For example:
+
+```
+...#......
+.......#..
+#.........
+..........
+......#...
+.#........
+.........#
+..........
+.......#..
+#...#.....
+
+```
+
+The researcher is trying to figure out the sum of the lengths of the *shortest path between every pair of galaxies*. However, there's a catch: the universe expanded in the time it took the light from those galaxies to reach the observatory.
+
+Due to something involving gravitational effects, *only some space expands*. In fact, the result is that *any rows or columns that contain no galaxies* should all actually be twice as big.
+
+In the above example, three columns and two rows contain no galaxies:
+
+```
+   v  v  v
+ ...#......
+ .......#..
+ #.........
+>..........<
+ ......#...
+ .#........
+ .........#
+>..........<
+ .......#..
+ #...#.....
+   ^  ^  ^
+
+```
+
+These rows and columns need to be *twice as big*; the result of cosmic expansion therefore looks like this:
+
+```
+....#........
+.........#...
+#............
+.............
+.............
+........#....
+.#...........
+............#
+.............
+.............
+.........#...
+#....#.......
+
+```
+
+Equipped with this expanded universe, the shortest path between every pair of galaxies can be found. It can help to assign every galaxy a unique number:
+
+```
+....1........
+.........2...
+3............
+.............
+.............
+........4....
+.5...........
+............6
+.............
+.............
+.........7...
+8....9.......
+
+```
+
+In these 9 galaxies, there are *36 pairs*. Only count each pair once; order within the pair doesn't matter. For each pair, find any shortest path between the two galaxies using only steps that move up, down, left, or right exactly one `.` or `#` at a time. (The shortest path between two galaxies is allowed to pass through another galaxy.)
+
+For example, here is one of the shortest paths between galaxies `5` and `9`:
+
+```
+....1........
+.........2...
+3............
+.............
+.............
+........4....
+.5...........
+.##.........6
+..##.........
+...##........
+....##...7...
+8....9.......
+
+```
+
+This path has length `*9*` because it takes a minimum of *nine steps* to get from galaxy `5` to galaxy `9` (the eight locations marked `#` plus the step onto galaxy `9` itself). Here are some other example shortest path lengths:
+
+* Between galaxy `1` and galaxy `7`: 15
+* Between galaxy `3` and galaxy `6`: 17
+* Between galaxy `8` and galaxy `9`: 5
+
+In this example, after expanding the universe, the sum of the shortest path between all 36 pairs of galaxies is `*374*`.
+
+Expand the universe, then find the length of the shortest path between every pair of galaxies. *What is the sum of these lengths?*
+
+Your puzzle answer was `10228230`.
+
+\--- Part Two ---
+----------
+
+The galaxies are much *older* (and thus much *farther apart*) than the researcher initially estimated.
+
+Now, instead of the expansion you did before, make each empty row or column *one million times* larger. That is, each empty row should be replaced with `1000000` empty rows, and each empty column should be replaced with `1000000` empty columns.
+
+(In the example above, if each empty row or column were merely `10` times larger, the sum of the shortest paths between every pair of galaxies would be `*1030*`. If each empty row or column were merely `100` times larger, the sum of the shortest paths between every pair of galaxies would be `*8410*`. However, your universe will need to expand far beyond these values.)
+
+Starting with the same initial image, expand the universe according to these new rules, then find the length of the shortest path between every pair of galaxies. *What is the sum of these lengths?*
+
+Your puzzle answer was `447073334102`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, you should [return to your Advent calendar](/2023) and try another puzzle.
+
+If you still want to see it, you can [get your puzzle input](11/input).