2020/23

y2020/p23.py

+import sys
+
+
+def play(start, cycles):
+    lim = len(start)
+    cur = start[0]
+    ns = [None for _ in range(lim + 1)]
+
+    for a, b in zip(start, start[1:] + start[:1]):
+        ns[a] = b
+
+    for _ in range(cycles):
+        a = ns[cur]
+        b = ns[a]
+        c = ns[b]
+
+        dest = (cur - 1) or lim
+        while dest in (a, b, c):
+            dest = (dest - 1) or lim
+
+        ns[dest], ns[c], ns[cur] = a, ns[dest], ns[c]
+        cur = ns[cur]
+
+    return ns
+
+
+text = sys.stdin.read().strip()
+ns = [int(n) for n in text]
+
+ordering = play(ns, 100)
+sol = [1]
+for _ in range(len(ordering) - 2):
+    sol.append(ordering[sol[-1]])
+print(''.join(str(n) for n in sol[1:]))
+
+ns.extend(n + 1 for n in range(len(ns), 1_000_000))
+ordering = play(ns, 10_000_000)
+print(ordering[1] * ordering[ordering[1]])