Add skip_list implementation

Author: Luke-Manyamazi

Sprint-2/implement_skip_list/skip_list.py

@@ -0,0 +1,94 @@
+import random
+
+class SkipListNode:
+    def __init__(self, value, level):
+        self.value = value
+        self.next = [None] * (level + 1)  # Next nodes at each level
+
+class SkipList:
+    def __init__(self):
+        self.max_level = 16
+        self.head = SkipListNode(None, self.max_level)  # Head node
+        self.level = 0  # Current highest level
+        self.size = 0
+    
+    def _random_level(self):
+        """Generate random level for a new node (coin flip)"""
+        level = 0
+        while random.random() < 0.5 and level < self.max_level:
+            level += 1
+        return level
+    
+    def __contains__(self, key):
+        """Support 'in' operator"""
+        return self.contains(key)
+    
+    def contains(self, key):
+        """Check if key exists in the skip list"""
+        current = self.head
+        
+        # Start from highest level and move down
+        for i in range(self.level, -1, -1):
+            while current.next[i] and current.next[i].value < key:
+                current = current.next[i]
+        
+        # Move to the node that might contain the value
+        current = current.next[0]
+        return current is not None and current.value == key
+    
+    def insert(self, value):
+        """Insert a value into the skip list"""
+        update = [None] * (self.max_level + 1)
+        current = self.head
+        
+        # Find the position to insert and track update pointers
+        for i in range(self.level, -1, -1):
+            while current.next[i] and current.next[i].value < value:
+                current = current.next[i]
+            update[i] = current
+        
+        current = current.next[0]
+        
+        # If value already exists, we'll still insert it (allow duplicates)
+        new_level = self._random_level()
+        
+        # Update the max level if needed
+        if new_level > self.level:
+            for i in range(self.level + 1, new_level + 1):
+                update[i] = self.head
+            self.level = new_level
+        
+        # Create new node
+        new_node = SkipListNode(value, new_level)
+        
+        # Update pointers at each level
+        for i in range(new_level + 1):
+            new_node.next[i] = update[i].next[i]
+            update[i].next[i] = new_node
+        
+        self.size += 1
+    
+    def to_list(self):
+        """Convert skip list to a sorted list"""
+        result = []
+        current = self.head.next[0]  # Start from first element at level 0
+        
+        while current:
+            result.append(current.value)
+            current = current.next[0]
+        
+        return result
+    
+    def __str__(self):
+        """String representation for debugging"""
+        result = []
+        for level in range(self.level, -1, -1):
+            level_str = f"Level {level}: "
+            current = self.head.next[level]
+            elements = []
+            while current:
+                elements.append(str(current.value))
+                current = current.next[level]
+            level_str += " -> ".join(elements) if elements else "empty"
+            result.append(level_str)
+        return "\n".join(result)