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+# --- Day 7: The Sum of Its Parts ---
+
+# You find yourself standing on a snow-covered coastline; apparently, you
+# landed a little off course. The region is too hilly to see the North Pole
+# from here, but you do spot some Elves that seem to be trying to unpack
+# something that washed ashore. It's quite cold out, so you decide to risk
+# creating a paradox by asking them for directions.
+
+# "Oh, are you the search party?" Somehow, you can understand whatever Elves
+# from the year 1018 speak; you assume it's Ancient Nordic Elvish. Could the
+# device on your wrist also be a translator? "Those clothes don't look very
+# warm; take this." They hand you a heavy coat.
+
+# "We do need to find our way back to the North Pole, but we have higher
+# priorities at the moment. You see, believe it or not, this box contains
+# something that will solve all of Santa's transportation problems - at least,
+# that's what it looks like from the pictures in the instructions." It doesn't
+# seem like they can read whatever language it's in, but you can: "Sleigh kit.
+# Some assembly required."
+
+# "'Sleigh'? What a wonderful name! You must help us assemble this 'sleigh' at
+# once!" They start excitedly pulling more parts out of the box.
+
+# The instructions specify a series of steps and requirements about which steps
+# must be finished before others can begin (your puzzle input). Each step is
+# designated by a single letter. For example, suppose you have the following
+# instructions:
+
+# Step C must be finished before step A can begin.
+# Step C must be finished before step F can begin.
+# Step A must be finished before step B can begin.
+# Step A must be finished before step D can begin.
+# Step B must be finished before step E can begin.
+# Step D must be finished before step E can begin.
+# Step F must be finished before step E can begin.
+
+# Visually, these requirements look like this:
+
+#   -->A--->B--
+#  /    \      \
+# C      -->D----->E
+#  \           /
+#   ---->F-----
+
+# Your first goal is to determine the order in which the steps should be
+# completed. If more than one step is ready, choose the step which is first
+# alphabetically. In this example, the steps would be completed as follows:
+
+#     Only C is available, and so it is done first.
+#     Next, both A and F are available. A is first alphabetically, so it is
+# done next.
+#     Then, even though F was available earlier, steps B and D are now also
+# available, and B is the first alphabetically of the three.
+#     After that, only D and F are available. E is not available because only
+# some of its prerequisites are complete. Therefore, D is completed next.
+#     F is the only choice, so it is done next.
+#     Finally, E is completed.
+
+# So, in this example, the correct order is CABDFE.
+
+# In what order should the steps in your instructions be completed?
+
+import networkx as nx
+
+with open("files/P7.txt") as f:
+    steps = [line for line in f.read().strip().split("\n")]
+
+
+def part_1() -> None:
+    G = nx.DiGraph()
+    for step in steps:
+        parent, child = step[5:6], step[36:37]
+        G.add_edge(parent, child)
+    order = "".join(nx.lexicographical_topological_sort(G))
+
+    print(f"The correct order will be {order}")
+
+
+if __name__ == "__main__":
+    part_1()