Python/data_structures/linked_list/singly_linked_list.py

class Node:  # create a Node
    def __init__(self, data):
        self.data = data  # given data
        self.next = None  # given next to None

    def __repr__(self):  # string representation of a Node
        return f"Node({self.data})"


class LinkedList:
    def __init__(self):
        self.head = None  # initialize head to None

    def insert_tail(self, data) -> None:
        if self.head is None:
            self.insert_head(data)  # if this is first node, call insert_head
        else:
            temp = self.head
            while temp.next:  # traverse to last node
                temp = temp.next
            temp.next = Node(data)  # create node & link to tail

    def insert_head(self, data) -> None:
        new_node = Node(data)  # create a new node
        if self.head:
            new_node.next = self.head  # link new_node to head
        self.head = new_node  # make NewNode as head

    def print_list(self) -> None:  # print every node data
        temp = self.head
        while temp:
            print(temp.data)
            temp = temp.next

    def delete_head(self):  # delete from head
        temp = self.head
        if self.head:
            self.head = self.head.next
            temp.next = None
        return temp

    def delete_tail(self):  # delete from tail
        temp = self.head
        if self.head:
            if self.head.next is None:  # if head is the only Node in the Linked List
                self.head = None
            else:
                while temp.next.next:  # find the 2nd last element
                    temp = temp.next
                # (2nd last element).next = None and temp = last element
                temp.next, temp = None, temp.next
        return temp

    def is_empty(self) -> bool:
        return self.head is None  # return True if head is none

    def reverse(self):
        prev = None
        current = self.head

        while current:
            # Store the current node's next node.
            next_node = current.next
            # Make the current node's next point backwards
            current.next = prev
            # Make the previous node be the current node
            prev = current
            # Make the current node the next node (to progress iteration)
            current = next_node
        # Return prev in order to put the head at the end
        self.head = prev

    def __repr__(self):  # String representation/visualization of a Linked Lists
        current = self.head
        string_repr = ""
        while current:
            string_repr += f"{current} --> "
            current = current.next
        # END represents end of the LinkedList
        return string_repr + "END"

    # Indexing Support. Used to get a node at particular position
    def __getitem__(self, index):
        current = self.head

        # If LinkedList is empty
        if current is None:
            raise IndexError("The Linked List is empty")

        # Move Forward 'index' times
        for _ in range(index):
            # If the LinkedList ends before reaching specified node
            if current.next is None:
                raise IndexError("Index out of range.")
            current = current.next
        return current

    # Used to change the data of a particular node
    def __setitem__(self, index, data):
        current = self.head
        # If list is empty
        if current is None:
            raise IndexError("The Linked List is empty")
        for i in range(index):
            if current.next is None:
                raise IndexError("Index out of range.")
            current = current.next
        current.data = data
       
    def __len__(self):
        """
        Return length of linked list i.e. number of nodes
        >>> linked_list = LinkedList()
        >>> len(linked_list)
        0
        >>> linked_list.insert_tail("head")
        >>> len(linked_list)
        1
        >>> linked_list.insert_head("head")
        >>> len(linked_list)
        2
        >>> _ = linked_list.delete_tail()
        >>> len(linked_list)
        1
        >>> _ = linked_list.delete_head()
        >>> len(linked_list)
        0
        """
        if not self.head:
            return 0

        count = 0
        cur_node = self.head
        while cur_node.next:
            count += 1
            cur_node = cur_node.next
        return count + 1


def main():
    A = LinkedList()
    A.insert_head(input("Inserting 1st at head ").strip())
    A.insert_head(input("Inserting 2nd at head ").strip())
    print("\nPrint list:")
    A.print_list()
    A.insert_tail(input("\nInserting 1st at tail ").strip())
    A.insert_tail(input("Inserting 2nd at tail ").strip())
    print("\nPrint list:")
    A.print_list()
    print("\nDelete head")
    A.delete_head()
    print("Delete tail")
    A.delete_tail()
    print("\nPrint list:")
    A.print_list()
    print("\nReverse linked list")
    A.reverse()
    print("\nPrint list:")
    A.print_list()
    print("\nString representation of linked list:")
    print(A)
    print("\nReading/changing Node data using indexing:")
    print(f"Element at Position 1: {A[1]}")
    A[1] = input("Enter New Value: ").strip()
    print("New list:")
    print(A)
    print(f"length of A is : {len(A)}")


if __name__ == "__main__":
    main()