From 26db82fd8560d54b1f385814b8871e6fba42fa91 Mon Sep 17 00:00:00 2001
From: louietyj <louietyj@gmail.com>
Date: Sat, 21 Oct 2017 10:39:55 +0800
Subject: [PATCH] Add Linked List (#71)

* LinkedList implementation

* LinkedList lazy iterator

* LinkedList example usage
---
 utilities/python/linked_list.py | 109 ++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 utilities/python/linked_list.py

diff --git a/utilities/python/linked_list.py b/utilities/python/linked_list.py
new file mode 100644
index 00000000..af0a7eb9
--- /dev/null
+++ b/utilities/python/linked_list.py
@@ -0,0 +1,109 @@
+# Singly-Linked List
+#
+# The linked list is passed around as a variable pointing to the
+# root node of the linked list, or None if the list is empty.
+
+class LinkedListNode:
+    def __init__(self, value):
+        self.value = value
+        self.next = None
+
+def linked_list_append(linked_list, value):
+    '''Appends a value to the end of the linked list'''
+    node = linked_list
+    insert_node = LinkedListNode(value)
+    if not node:
+        return insert_node
+    while node.next:
+        node = node.next
+    node.next = insert_node
+    return linked_list
+
+def linked_list_insert_index(linked_list, value, index):
+    '''Inserts a value at a particular index'''
+    node = linked_list
+    insert_node = LinkedListNode(value)
+    
+    # Check if inserting at head
+    if index == 0:
+        insert_node.next = node
+        return insert_node
+
+    # Skip ahead
+    for _ in range(index - 1):
+        node = node.next
+        if not node:
+            raise ValueError
+    insert_node.next = node.next
+    node.next = insert_node
+    return linked_list
+
+def linked_list_delete(linked_list, value):
+    '''Deletes the first occurrence of a value in the linked list'''
+    node = linked_list
+    
+    # Check if deleting at head
+    if node.value == value:
+        return node.next
+
+    # Skip ahead
+    while node.next:
+        if node.next.value == value:
+            node.next = node.next.next
+            return linked_list
+        node = node.next
+    raise ValueError
+
+def linked_list_delete_index(linked_list, index):
+    '''Deletes the element at a particular index in the linked list'''
+    node = linked_list
+    
+    # Check if deleting at head
+    if index == 0:
+        return node.next
+
+    # Skip ahead
+    for _ in range(index - 1):
+        node = node.next
+        if not node:
+            raise ValueError
+    if not node.next:
+        raise ValueError
+    node.next = node.next.next
+    return linked_list
+
+def linked_list_iter(linked_list):
+    '''Lazy iterator over each node in the linked list'''
+    node = linked_list
+    while node is not None:
+        yield node
+        node = node.next
+
+
+# Append to back
+linked_list = None    # Start with an empty linked list
+linked_list = linked_list_append(linked_list, 1)
+linked_list = linked_list_append(linked_list, 2)
+linked_list = linked_list_append(linked_list, 4)
+print([node.value for node in linked_list_iter(linked_list)])
+
+# Insert by index
+linked_list = linked_list_insert_index(linked_list, 0, 0) # Front
+print([node.value for node in linked_list_iter(linked_list)])
+linked_list = linked_list_insert_index(linked_list, 3, 3) # Back
+print([node.value for node in linked_list_iter(linked_list)])
+
+# Delete "3"
+linked_list = linked_list_delete(linked_list, 3)
+print([node.value for node in linked_list_iter(linked_list)])
+
+# Delete by index
+linked_list = linked_list_delete_index(linked_list, 0)
+print([node.value for node in linked_list_iter(linked_list)])
+linked_list = linked_list_delete_index(linked_list, 1)
+print([node.value for node in linked_list_iter(linked_list)])
+
+# Delete until empty
+linked_list = linked_list_delete_index(linked_list, 0)
+linked_list = linked_list_delete_index(linked_list, 0)
+print([node.value for node in linked_list_iter(linked_list)])