add day 7 info

src/examples/07.txt

@@ -0,0 +1,23 @@
+$ cd /
+$ ls
+dir a
+14848514 b.txt
+8504156 c.dat
+dir d
+$ cd a
+$ ls
+dir e
+29116 f
+2557 g
+62596 h.lst
+$ cd e
+$ ls
+584 i
+$ cd ..
+$ cd ..
+$ cd d
+$ ls
+4060174 j
+8033020 d.log
+5626152 d.ext
+7214296 k

src/puzzles/07.md

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+\--- Day 7: No Space Left On Device ---
+----------
+
+You can hear birds chirping and raindrops hitting leaves as the expedition proceeds. Occasionally, you can even hear much louder sounds in the distance; how big do the animals get out here, anyway?
+
+The device the Elves gave you has problems with more than just its communication system. You try to run a system update:
+
+```
+$ system-update --please --pretty-please-with-sugar-on-top
+Error: No space left on device
+
+```
+
+Perhaps you can delete some files to make space for the update?
+
+You browse around the filesystem to assess the situation and save the resulting terminal output (your puzzle input). For example:
+
+```
+$ cd /
+$ ls
+dir a
+14848514 b.txt
+8504156 c.dat
+dir d
+$ cd a
+$ ls
+dir e
+29116 f
+2557 g
+62596 h.lst
+$ cd e
+$ ls
+584 i
+$ cd ..
+$ cd ..
+$ cd d
+$ ls
+4060174 j
+8033020 d.log
+5626152 d.ext
+7214296 k
+
+```
+
+The filesystem consists of a tree of files (plain data) and directories (which can contain other directories or files). The outermost directory is called `/`. You can navigate around the filesystem, moving into or out of directories and listing the contents of the directory you're currently in.
+
+Within the terminal output, lines that begin with `$` are *commands you executed*, very much like some modern computers:
+
+* `cd` means *change directory*. This changes which directory is the current directory, but the specific result depends on the argument:
+  * `cd x` moves *in* one level: it looks in the current directory for the directory named `x` and makes it the current directory.
+  * `cd ..` moves *out* one level: it finds the directory that contains the current directory, then makes that directory the current directory.
+  * `cd /` switches the current directory to the outermost directory, `/`.
+
+* `ls` means *list*. It prints out all of the files and directories immediately contained by the current directory:
+  * `123 abc` means that the current directory contains a file named `abc` with size `123`.
+  * `dir xyz` means that the current directory contains a directory named `xyz`.
+
+Given the commands and output in the example above, you can determine that the filesystem looks visually like this:
+
+```
+- / (dir)
+  - a (dir)
+    - e (dir)
+      - i (file, size=584)
+    - f (file, size=29116)
+    - g (file, size=2557)
+    - h.lst (file, size=62596)
+  - b.txt (file, size=14848514)
+  - c.dat (file, size=8504156)
+  - d (dir)
+    - j (file, size=4060174)
+    - d.log (file, size=8033020)
+    - d.ext (file, size=5626152)
+    - k (file, size=7214296)
+
+```
+
+Here, there are four directories: `/` (the outermost directory), `a` and `d` (which are in `/`), and `e` (which is in `a`). These directories also contain files of various sizes.
+
+Since the disk is full, your first step should probably be to find directories that are good candidates for deletion. To do this, you need to determine the *total size* of each directory. The total size of a directory is the sum of the sizes of the files it contains, directly or indirectly. (Directories themselves do not count as having any intrinsic size.)
+
+The total sizes of the directories above can be found as follows:
+
+* The total size of directory `e` is *584* because it contains a single file `i` of size 584 and no other directories.
+* The directory `a` has total size *94853* because it contains files `f` (size 29116), `g` (size 2557), and `h.lst` (size 62596), plus file `i` indirectly (`a` contains `e` which contains `i`).
+* Directory `d` has total size *24933642*.
+* As the outermost directory, `/` contains every file. Its total size is *48381165*, the sum of the size of every file.
+
+To begin, find all of the directories with a total size of *at most 100000*, then calculate the sum of their total sizes. In the example above, these directories are `a` and `e`; the sum of their total sizes is `*95437*` (94853 + 584). (As in this example, this process can count files more than once!)
+
+Find all of the directories with a total size of at most 100000. *What is the sum of the total sizes of those directories?*
+
+To begin, [get your puzzle input](7/input).
+
+Answer:
+
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