Added Solution for 2021 day 09

2021/day09_smoke_basin/challenge.txt

@@ -0,0 +1,91 @@
+These caves seem to be [lava tubes](https://en.wikipedia.org/wiki/Lava_tube). Parts are even still volcanically active; small hydrothermal vents release smoke into the caves that slowly settles like rain.
+
+If you can model how the smoke flows through the caves, you might be able to avoid it and be that much safer. The submarine generates a heightmap of the floor of the nearby caves for you (your puzzle input).
+
+Smoke flows to the lowest point of the area it's in. For example, consider the following heightmap:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+Each number corresponds to the height of a particular location, where `9` is the highest and `0` is the lowest a location can be.
+
+Your first goal is to find the *low points* - the locations that are lower than any of its adjacent locations. Most locations have four adjacent locations (up, down, left, and right); locations on the edge or corner of the map have three or two adjacent locations, respectively. (Diagonal locations do not count as adjacent.)
+
+In the above example, there are *four* low points, all highlighted: two are in the first row (a `1` and a `0`), one is in the third row (a `5`), and one is in the bottom row (also a `5`). All other locations on the heightmap have some lower adjacent location, and so are not low points.
+
+The *risk level* of a low point is *1 plus its height*. In the above example, the risk levels of the low points are `2`, `1`, `6`, and `6`. The sum of the risk levels of all low points in the heightmap is therefore `*15*`.
+
+Find all of the low points on your heightmap. *What is the sum of the risk levels of all low points on your heightmap?*
+
+Your puzzle answer was `524`.
+
+\--- Part Two ---
+----------
+
+Next, you need to find the largest basins so you know what areas are most important to avoid.
+
+A *basin* is all locations that eventually flow downward to a single low point. Therefore, every low point has a basin, although some basins are very small. Locations of height `9` do not count as being in any basin, and all other locations will always be part of exactly one basin.
+
+The *size* of a basin is the number of locations within the basin, including the low point. The example above has four basins.
+
+The top-left basin, size `3`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The top-right basin, size `9`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The middle basin, size `14`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The bottom-right basin, size `9`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+Find the three largest basins and multiply their sizes together. In the above example, this is `9 * 14 * 9 = *1134*`.
+
+*What do you get if you multiply together the sizes of the three largest basins?*
+
+Your puzzle answer was `1235430`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, you should [return to your Advent calendar](/2021) and try another puzzle.
+
+If you still want to see it, you can [get your puzzle input](9/input).