implemented skip list

Author: donarbl

Sprint-2/implement_skip_list/skip_list.py

@@ -0,0 +1,74 @@
+import random
+
+
+class _Node:
+    def __init__(self, value, level):
+        self.value = value
+        # forward[i] is the next node at level i
+        self.forward = [None] * (level + 1)
+
+
+class SkipList:
+    MAX_LEVEL = 16
+    P = 0.5  # probability for level increase
+
+    def __init__(self):
+        # head is a dummy node with no value but MAX_LEVEL pointers
+        self.head = _Node(None, self.MAX_LEVEL)
+        self.level = 0  # current highest level in use
+        self.size = 0
+
+    def _random_level(self):
+        lvl = 0
+        while random.random() < self.P and lvl < self.MAX_LEVEL:
+            lvl += 1
+        return lvl
+
+    def insert(self, value):
+        """Insert value in sorted order."""
+        update = [None] * (self.MAX_LEVEL + 1)
+        current = self.head
+
+        # 1) Search for position from top level down
+        for lvl in range(self.level, -1, -1):
+            while current.forward[lvl] is not None and current.forward[lvl].value < value:
+                current = current.forward[lvl]
+            update[lvl] = current
+
+        # 2) Check if already present -> do nothing
+        next_node = current.forward[0]
+        if next_node is not None and next_node.value == value:
+            return
+
+        # 3) Choose random level for new node
+        new_level = self._random_level()
+        if new_level > self.level:
+            for i in range(self.level + 1, new_level + 1):
+                update[i] = self.head
+            self.level = new_level
+
+        # 4) Insert node, fixing pointers at each level
+        new_node = _Node(value, new_level)
+        for i in range(new_level + 1):
+            new_node.forward[i] = update[i].forward[i]
+            update[i].forward[i] = new_node
+
+        self.size += 1
+
+    def __contains__(self, value):
+        """Implements `value in sl` using fast skip-list search."""
+        current = self.head
+        for lvl in range(self.level, -1, -1):
+            while current.forward[lvl] is not None and current.forward[lvl].value < value:
+                current = current.forward[lvl]
+        current = current.forward[0]
+        return current is not None and current.value == value
+
+    def to_list(self):
+        """Return all values in sorted order from the bottom level."""
+        result = []
+        current = self.head.forward[0]
+        while current is not None:
+            result.append(current.value)
+            current = current.forward[0]
+        return result