From f92c2ee6bc0453b30cc438e8e9314eb1730457ec Mon Sep 17 00:00:00 2001
From: Wenru Dong <dongwenru.aaron.shenyang@gmail.com>
Date: Sun, 28 Oct 2018 20:46:23 +0000
Subject: [PATCH] Implementation of SkipList in python

---
 python/17_skiplist/skip_list.py | 91 +++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 python/17_skiplist/skip_list.py

diff --git a/python/17_skiplist/skip_list.py b/python/17_skiplist/skip_list.py
new file mode 100644
index 00000000..52069eb1
--- /dev/null
+++ b/python/17_skiplist/skip_list.py
@@ -0,0 +1,91 @@
+"""
+    An implementation of skip list.
+    The list stores positive integers without duplicates.
+
+    跳表的一种实现方法。
+    跳表中储存的是正整数，并且储存的是不重复的。
+
+    Author: Wenru
+"""
+
+from typing import Optional
+import random
+
+class ListNode:
+
+    def __init__(self, data: Optional[int] = None):
+        self._data = data
+        self._forwards = []   # Forward pointers
+
+class SkipList:
+
+    _MAX_LEVEL = 16
+
+    def __init__(self):
+        self._level_count = 1
+        self._head = ListNode()
+        self._head._forwards = [None] * type(self)._MAX_LEVEL
+
+    def find(self, value: int) -> Optional[ListNode]:
+        p = self._head
+        for i in range(self._level_count - 1, -1, -1):   # Move down a level
+            while p._forwards[i] and p._forwards[i]._data < value:
+                p = p._forwards[i]   # Move along level
+        
+        return p._forwards[0] if p._forwards[0] and p._forwards[0]._data == value else None
+
+    def insert(self, value: int):
+        level = self._random_level()
+        if self._level_count < level: self._level_count = level
+        new_node = ListNode(value)
+        new_node._forwards = [None] * level
+        update = [self._head] * level     # update is like a list of prevs
+
+        p = self._head
+        for i in range(level - 1, -1, -1):
+            while p._forwards[i] and p._forwards[i]._data < value:
+                p = p._forwards[i]
+            
+            update[i] = p     # Found a prev
+
+        for i in range(level):
+            new_node._forwards[i] = update[i]._forwards[i]   # new_node.next = prev.next
+            update[i]._forwards[i] = new_node     # prev.next = new_node
+        
+    def delete(self, value):
+        update = [None] * self._level_count
+        p = self._head
+        for i in range(self._level_count - 1, -1, -1):
+            while p._forwards[i] and p._forwards[i]._data < value:
+                p = p._forwards[i]
+            update[i] = p
+        
+        if p._forwards[0] and p._forwards[0]._data == value:
+            for i in range(self._level_count - 1, -1, -1):
+                if update[i]._forwards[i] and update[i]._forwards[i]._data == value:
+                    update[i]._forwards[i] = update[i]._forwards[i]._forwards[i]     # Similar to prev.next = prev.next.next
+
+    def _random_level(self, p: float = 0.5) -> int:
+        level = 1
+        while random.random() < p and level < type(self)._MAX_LEVEL:
+            level += 1
+        return level
+
+    def __repr__(self) -> str:
+        values = []
+        p = self._head
+        while p._forwards[0]:
+            values.append(str(p._forwards[0]._data))
+            p = p._forwards[0]
+        return "->".join(values)
+
+
+if __name__ == "__main__":
+    l = SkipList()
+    for i in range(10):
+        l.insert(i)
+    print(l)
+    p = l.find(7)
+    print(p._data)
+    l.delete(3)
+    print(l)
\ No newline at end of file