Linked List

Author: hashfx

DSA-Python/DataStructures/LinkedList.py

@@ -0,0 +1,144 @@
+"""
+Linked Lists:
+    Single Linked Lists:
+        |data|LinkToNextData|--->|data|LinkToNextData|--->|data|null|
+        (disadvantage: can't go back to previous data after one data point)
+
+    Double Linked List:
+        |Null|data|LinkToNextData|<--->|LinkToPreviousData|data|LinkToNextData|<--->|LinkToPreviousData|data|null|
+
+    Advantage of Linked List over array:
+        > No need to pre-allocate the space
+        > Insertion is easier
+
+    BigO of Linked List:
+        Indexing -----> O(n)
+        Insert/Delete element at Start -----> O(1)
+        Insert/Delete element at End -----> O(n)
+        Insert element at Middle -----> O(n)
+        Linked List Traversal(Reverse) -----> O(n)
+        Accessing Elements by Value -----> O(n)
+"""
+
+class Node:
+    '''
+    Working of Class Node:
+        whenever this class is called, a data will be passed which will get saved in LinkedList as a node which has 2 parts: data & next
+    '''
+    def __init__(self, data=None, next=None):  # initialised data to None
+        self.data = data
+        self.next = next
+
+class LinkedList():
+    def __init__(self):
+        self.head = Node()  # Head is starting point of a Linked List and points to no other value
+
+    # def accept_data(self, data):
+    def accept_data(self, *data): # args to get data at single go
+        new_node = Node(data)  # created a new node to add data
+        cur = self.head  # pointer set for the next data
+        while cur.next != None:
+            # let linked list have 5 elements; new node will be added as 6th node in linkedlistl;
+            # so we will have to search for the last pointer (ptr of 5th ele)
+            # so we will iterate until ptr of of curent ele becomes none; as it gets None we will come to know that this is the last node
+            cur = cur.next
+        cur.next = new_node  # pointer set to the new node
+
+
+    # method to print each element of Linked List
+    def display(self):
+        disp = []
+        cur = self.head
+        disp.append(cur.data)  # to display head element
+
+        while cur.next != None:  # will iterate until next val of current pointer gets None
+            cur = cur.next  # accessing next element of the node
+            disp.append(cur.data)  # appended the data of current element to the list disp[]
+        print(disp)
+
+    # Method to get length of elements in Linked List
+    def getLength(self):
+        count = 0
+        itr = self.head
+        while itr:
+            count += 1
+            itr = itr.next
+        return count
+
+    def removeAtIndex(self, index):
+        # check if index is valid
+        if index < 0 or index >= self.getLength():
+            raise Exception("Invalid Index")
+
+        if index == 0:  # removing head
+            self.head = self.head.next
+            return
+
+        count = 0
+        itr = self.head
+        while itr:
+            # removing element at given index
+            if count == index - 1:  # we need to stop at the element prior to element we are removing
+                # After element(e1) is removed, address of e2 is linked to e0
+                itr.next = itr.next.next
+                break
+            itr = itr.next
+            count += 1
+
+
+    def insertAtIndex(self, index, data):
+        # check if index is valid
+        if index < 0 or index >= self.getLength():
+            raise Exception("Invalid Index")
+
+        if index == 0:  # value is inserted at index[0]
+            self.accept_data(data)
+            return
+
+        count = 0
+        itr = self.head
+        while itr:
+            if count == index - 1:
+                node = Node(data, itr.next)
+                itr.next = node
+            itr = itr.next
+            count+=1
+
+
+
+l = LinkedList()  # created an object 'l' to call class LinkedList
+
+# adding elements to linked list one-by-one
+
+l.head = Node(1)  # Head of the Linked List
+element2 = Node(2)
+element3 = Node(3)
+element4 = Node(4)
+
+# Linking Nodes of the Linked List to each other
+# if not linked, only value at head will be printed
+l.head.next = element2  # head is pointing to element2
+element2.next = element3  # element2 is pointing to element3
+element3.next = element4  # element3 is pointing to element4
+
+# display Linked List
+l.display()
+print("Length: ", l.getLength())
+l.removeAtIndex(2)
+l.display()
+l.insertAtIndex(2, 3)  # (index, value)
+l.display()
+
+# adding elements of LinkedList at once
+# l = LinkedList()
+# l.accept_data(1,2,3,4,5)
+
+
+# if function accept_data is used without (*data) as parameter
+# adding values one-by-one
+# l.accept_data(1)
+# l.accept_data(2)
+# l.accept_data(3)
+# l.accept_data(4)
+# l.accept_data(5)
+# l.display()