implement skip list

Author: PratiAmalden

Sprint-2/implement_skip_list/skip_list.py

@@ -0,0 +1,71 @@
+import random
+
+
+class Node:
+    def __init__(self, value, level):
+        self.value = value
+        self.next = [None] * (level + 1) # next[i] the next node at level i
+
+
+class SkipList:
+    MAX_LEVEL = 16
+    P = 0.5
+
+    def __init__(self):
+        self.head = Node(None, self.MAX_LEVEL)
+        self.level = 0          # highest level currently in use
+
+    def _random_level(self):
+        lvl = 0
+        while random.random() < self.P and lvl < self.MAX_LEVEL:
+            lvl += 1
+        return lvl
+
+    def insert(self, value):
+        update = [None] * (self.MAX_LEVEL + 1)
+        x = self.head
+
+        # Find the place where the new value should go
+        for i in range(self.level, -1, -1):
+            while x.next[i] and x.next[i].value < value:
+                x = x.next[i]
+            update[i] = x
+
+        x0 = x.next[0]
+        if x0 and x0.value == value:
+            return
+
+        node_level = self._random_level()
+
+        if node_level > self.level:
+            for i in range(self.level + 1, node_level + 1):
+                update[i] = self.head
+            self.level = node_level
+
+        new_node = Node(value, node_level)
+        for i in range(node_level + 1):
+            new_node.next[i] = update[i].next[i]
+            update[i].next[i] = new_node
+    
+    # Check if value exists
+    def contains(self, value):
+        x = self.head
+        for lvl in range(self.level, -1, -1):
+            while x.next[lvl] and x.next[lvl].value < value:
+                x = x.next[lvl]
+
+        x = x.next[0]
+        return x is not None and x.value == value
+
+    def __contains__(self, value):
+        return self.contains(value)
+
+    def to_list(self):
+        result = []
+        x = self.head.next[0]
+        while x:
+            result.append(x.value)
+            x = x.next[0]
+        return result
+
+