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y2019/p06.py

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+import sys
+import collections
+
+
+# build
+text = sys.stdin.read()
+directed = {}
+undirected = collections.defaultdict(set)
+for line in text.splitlines():
+    A, B = line.split(')')
+    directed[B] = A
+    undirected[B].add(A)
+    undirected[A].add(B)
+
+# pop stack
+i = 0
+for k in directed:
+    while k in directed:
+        k = directed[k]
+        i += 1
+print(i)
+
+# bfs
+hops = 0
+seen = set()
+edge = {'YOU'}
+while 'SAN' not in edge:
+    hops += 1
+    seen |= edge
+    edge = {new for node in edge for new in undirected[node]} - seen
+print(hops - 2)