Solve clinic

clinic.py

@@ -0,0 +1,93 @@
+""" https://coderun.yandex.ru/problem/goblins-and-chess """
+
+
+class Node:
+    """Double-linked list node"""
+
+    def __init__(self, value=None):
+        self.value = value
+        self.next = None
+        self.prev = None
+
+    def connect(self, next=None, prev=None):
+        self.next = next
+        self.prev = prev
+        if next:
+            next.prev = self
+        if prev:
+            prev.next = self
+
+    def walk(self):
+        node = self
+        while node:
+            yield str(node.value)
+            node = node.next
+
+
+class Pointer:
+    """Pointer to the node together with its place in the queue"""
+
+    def __init__(self, node: Node, index: int):
+        self.node = node
+        self.index = index
+
+    def move(self, target: int):
+        while target > self.index:
+            self.node = self.node.next
+            self.index += 1
+        while target < self.index:
+            self.node = self.node.prev
+            self.index -= 1
+
+    def correct(self, delta):
+        self.index += delta
+
+    def __str__(self):
+        return f"[#{self.index}] " + "->".join(self.node.walk())
+
+
+def main():
+    """Solve the clinic problem with three pointers"""
+    N = int(input())
+
+    head = Pointer(Node(), 0)
+    middle = Pointer(head.node, 0)
+    tail = Pointer(Node(), 1)
+    tail.node.connect(prev=head.node)
+
+    # middle points to the center as defined by index len(tail) // 2
+    # 0 elements: tail=1, middle=head=0
+    # 1 element: tail=2, middle=1, head=0
+    # 2 elements: tail=3, 2, middle=1, head=0
+    # we always insert between tail and middle
+
+    for _ in range(N):
+        s = input()
+        # print(head, middle, s, sep="  |  ")
+        if s[0] == "-":
+            del_node = head.node.next
+            print(del_node.value)
+            head.node.connect(next=del_node.next)
+            if middle.index == 1:
+                assert del_node == middle.node
+                middle = Pointer(head.node.next, 1)
+            else:
+                assert del_node != middle.node
+                middle.correct(-1)
+            tail.correct(-1)
+        else:
+            new_node = Node(int(s.split()[1].strip()))
+            if s[0] == "*":
+                # insert after the middle
+                new_node.connect(prev=middle.node, next=middle.node.next)
+            else:
+                assert s[0] == "+"
+                # insert before the tail
+                new_node.connect(prev=tail.node.prev, next=tail.node)
+            tail.correct(+1)
+
+        middle.move(tail.index // 2)
+
+
+if __name__ == "__main__":
+    main()