From 7376addcd5f786a3c46e48e5600cf88d363015c3 Mon Sep 17 00:00:00 2001
From: Mariusz Skoneczko <mariusz@skoneczko.com>
Date: Fri, 18 Oct 2019 17:38:31 +1100
Subject: [PATCH] Implement Linked Queue and Linked Stack data structures
 (#1324)

* Add LinkedQueue

* Add LinkedStack
---
 data_structures/queue/linked_queue.py  | 74 ++++++++++++++++++++++++++
 data_structures/stacks/linked_stack.py | 67 +++++++++++++++++++++++
 2 files changed, 141 insertions(+)
 create mode 100644 data_structures/queue/linked_queue.py
 create mode 100644 data_structures/stacks/linked_stack.py

diff --git a/data_structures/queue/linked_queue.py b/data_structures/queue/linked_queue.py
new file mode 100644
index 000000000..614c60cd1
--- /dev/null
+++ b/data_structures/queue/linked_queue.py
@@ -0,0 +1,74 @@
+""" A Queue using a Linked List like structure """
+from typing import Any, Optional
+
+
+class Node:
+    def __init__(self, data: Any, next: Optional["Node"] = None):
+        self.data: Any = data
+        self.next: Optional["Node"] = next
+
+
+class LinkedQueue:
+    """
+    Linked List Queue implementing put (to end of queue),
+    get (from front of queue) and is_empty
+
+    >>> queue = LinkedQueue()
+    >>> queue.is_empty()
+    True
+    >>> queue.put(5)
+    >>> queue.put(9)
+    >>> queue.put('python')
+    >>> queue.is_empty();
+    False
+    >>> queue.get()
+    5
+    >>> queue.put('algorithms')
+    >>> queue.get()
+    9
+    >>> queue.get()
+    'python'
+    >>> queue.get()
+    'algorithms'
+    >>> queue.is_empty()
+    True
+    >>> queue.get()
+    Traceback (most recent call last):
+        ...
+    IndexError: get from empty queue
+    """
+
+    def __init__(self) -> None:
+        self.front: Optional[Node] = None
+        self.rear: Optional[Node] = None
+
+    def is_empty(self) -> bool:
+        """ returns boolean describing if queue is empty """
+        return self.front is None
+
+    def put(self, item: Any) -> None:
+        """ append item to rear of queue """
+        node: Node = Node(item)
+        if self.is_empty():
+            # the queue contains just the single element
+            self.front = node
+            self.rear = node
+        else:
+            # not empty, so we add it to the rear of the queue
+            assert isinstance(self.rear, Node)
+            self.rear.next = node
+            self.rear = node
+
+    def get(self) -> Any:
+        """ returns and removes item at front of queue """
+        if self.is_empty():
+            raise IndexError("get from empty queue")
+        else:
+            # "remove" element by having front point to the next one
+            assert isinstance(self.front, Node)
+            node: Node = self.front
+            self.front = node.next
+            if self.front is None:
+                self.rear = None
+
+            return node.data
diff --git a/data_structures/stacks/linked_stack.py b/data_structures/stacks/linked_stack.py
new file mode 100644
index 000000000..18ba87ddc
--- /dev/null
+++ b/data_structures/stacks/linked_stack.py
@@ -0,0 +1,67 @@
+""" A Stack using a Linked List like structure """
+from typing import Any, Optional
+
+
+class Node:
+    def __init__(self, data: Any, next: Optional["Node"] = None):
+        self.data: Any = data
+        self.next: Optional["Node"] = next
+
+
+class LinkedStack:
+    """
+    Linked List Stack implementing push (to top),
+    pop (from top) and is_empty
+
+    >>> stack = LinkedStack()
+    >>> stack.is_empty()
+    True
+    >>> stack.push(5)
+    >>> stack.push(9)
+    >>> stack.push('python')
+    >>> stack.is_empty();
+    False
+    >>> stack.pop()
+    'python'
+    >>> stack.push('algorithms')
+    >>> stack.pop()
+    'algorithms'
+    >>> stack.pop()
+    9
+    >>> stack.pop()
+    5
+    >>> stack.is_empty()
+    True
+    >>> stack.pop()
+    Traceback (most recent call last):
+        ...
+    IndexError: pop from empty stack
+    """
+
+    def __init__(self) -> None:
+        self.top: Optional[Node] = None
+
+    def is_empty(self) -> bool:
+        """ returns boolean describing if stack is empty """
+        return self.top is None
+
+    def push(self, item: Any) -> None:
+        """ append item to top of stack """
+        node: Node = Node(item)
+        if self.is_empty():
+            self.top = node
+        else:
+            # each node points to the item "lower" in the stack
+            node.next = self.top
+            self.top = node
+
+    def pop(self) -> Any:
+        """ returns and removes item at top of stack """
+        if self.is_empty():
+            raise IndexError("pop from empty stack")
+        else:
+            # "remove" element by having top point to the next one
+            assert isinstance(self.top, Node)
+            node: Node = self.top
+            self.top = node.next
+            return node.data