GH-90: Add Python Linked List (#103)

Author: quang-ph

README.md

@@ -53,6 +53,7 @@ A data structure is a data organization, management, and storage format that is
 <tr>
 <td>Linked List</td>
 <td>
+<a href="/concepts/python/linked-list.md"><code>py</code></a>
 <a href="/concepts/typescript/linked-list.md"><code>ts</code></a>
 </td>
 <tr>

concepts/python/linked_list.md

@@ -0,0 +1,98 @@
+# Linked List
+
+From wikipedia
+
+> In computer science, a linked list is a linear collection of data elements whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes which together represent a sequence.
+
+## Time Complexity Analysis of Linked List
+
+INSERTION
+
+| Operation       | Singly Linked List  | Double Linked List  |
+|-----------------|---------------------|---------------------|
+| Insert head     | $O(1)$              | $O(1)$              |
+| Insert middle   | $O(n)$              | $O(n)$              |
+| Insert tail     | $O(n)$              | $O(1)$              |
+
+DELETION
+
+| Operation       | Singly Linked List  | Double Linked List  |
+|-----------------|---------------------|---------------------|
+| Delete head     | $O(1)$              | $O(1)$              |
+| Delete middle   | $O(n)$              | $O(n)$              |
+| Delete tail     | $O(n)$              | $O(1)$              |
+
+SEARCHING
+
+| Singly Linked List  | Double Linked List  |
+|---------------------|---------------------|
+| $O(n)$              | $O(n)$              |
+
+## Singly Linked List Implementation in Python
+
+```python
+# Implement using collections.deque
+from collections import deque
+
+
+linked_list = deque(['a','b','c','d','e'])  # a -> b -> c -> d -> e -> None
+linked_list.append('f')                     # a -> b -> c -> d -> e -> f -> None
+linked_list.pop()                           # a -> b -> c -> d -> e -> None
+linked_list.appendleft('z')                 # z -> a -> b -> c -> d -> e -> None
+linked_list.popleft()                       # a -> b -> c -> d -> e -> None
+
+# Implement from scratch
+class LinkedList:
+    def __init__(self, nodes=None):
+        self.head = None
+        if nodes is not None:
+            node = Node(data=nodes.pop(0))
+            self.head = node
+            for item in nodes:
+                node.next = Node(data=item)
+                node = node.next
+
+    def __iter__(self):
+        node = self.head
+        while node is not None:
+            yield node
+            node = node.next
+
+    def __repr__(self):
+        node = self.head
+        nodes = []
+        while node is not None:
+            nodes.append(node.data)
+            node = node.next
+        nodes.append("None")
+        return " -> ".join(nodes)
+
+    def add_first(self, node):
+        node.next = self.head
+        self.head = node
+
+    def add_last(self, node):
+        if self.head is None:
+            self.head = node
+            return
+        for current_node in self:
+            pass
+        current_node.next = node
+
+class Node:
+    def __init__(self, data):
+        self.data = data
+        self.next = None
+
+    def __repr__(self):
+        return self.data
+
+linked_list = LinkedList(['b','c','d','e']) # b -> c -> d -> e -> None
+linked_list.add_first(Node("a"))            # a -> b -> c -> d -> e -> None
+linked_list.add_last(Node("f"))             # a -> b -> c -> d -> e -> f -> None
+```
+## 🔗 Further Reading
+
+* [Linked Lists in Python](https://realpython.com/linked-lists-python/), realpython.com
+* ▶️ [Linked Lists Introduction](https://www.youtube.com/watch?v=-Yn5DU0_-lw&t=7s&ab_channel=WilliamFiset), WilliamFiset, 2017
+* ▶️ [CS50 2018 - Lecture 4 - Linked Lists](https://www.youtube.com/watch?v=wh4TS7RJDTA), CS50, 2018