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Singly Linked List

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from typing import Any


class Node:
    def __init__(self, data: Any):
        """
        Create and initialize Node class instance.
        >>> Node(20)
        Node(20)
        >>> Node("Hello, world!")
        Node(Hello, world!)
        >>> Node(None)
        Node(None)
        >>> Node(True)
        Node(True)
        """
        self.data = data
        self.next = None

    def __repr__(self) -> str:
        """
        Get the string representation of this node.
        >>> Node(10).__repr__()
        'Node(10)'
        """
        return f"Node({self.data})"


class LinkedList:
    def __init__(self):
        """
        Create and initialize LinkedList class instance.
        >>> linked_list = LinkedList()
        """
        self.head = None

    def __iter__(self) -> Any:
        """
        This function is intended for iterators to access
        and iterate through data inside linked list.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("tail")
        >>> linked_list.insert_tail("tail_1")
        >>> linked_list.insert_tail("tail_2")
        >>> for node in linked_list: # __iter__ used here.
        ...     node
        'tail'
        'tail_1'
        'tail_2'
        """
        node = self.head
        while node:
            yield node.data
            node = node.next

    def __len__(self) -> int:
        """
        Return length of linked list i.e. number of nodes
        >>> linked_list = LinkedList()
        >>> len(linked_list)
        0
        >>> linked_list.insert_tail("tail")
        >>> 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
        """
        return len(tuple(iter(self)))

    def __repr__(self) -> str:
        """
        String representation/visualization of a Linked Lists
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail(1)
        >>> linked_list.insert_tail(3)
        >>> linked_list.__repr__()
        '1->3'
        """
        return "->".join([str(item) for item in self])

    def __getitem__(self, index: int) -> Any:
        """
        Indexing Support. Used to get a node at particular position
        >>> linked_list = LinkedList()
        >>> for i in range(0, 10):
        ...     linked_list.insert_nth(i, i)
        >>> all(str(linked_list[i]) == str(i) for i in range(0, 10))
        True
        >>> linked_list[-10]
        Traceback (most recent call last):
            ...
        ValueError: list index out of range.
        >>> linked_list[len(linked_list)]
        Traceback (most recent call last):
            ...
        ValueError: list index out of range.
        """
        if not 0 <= index < len(self):
            raise ValueError("list index out of range.")
        for i, node in enumerate(self):
            if i == index:
                return node

    # Used to change the data of a particular node
    def __setitem__(self, index: int, data: Any) -> None:
        """
        >>> linked_list = LinkedList()
        >>> for i in range(0, 10):
        ...     linked_list.insert_nth(i, i)
        >>> linked_list[0] = 666
        >>> linked_list[0]
        666
        >>> linked_list[5] = -666
        >>> linked_list[5]
        -666
        >>> linked_list[-10] = 666
        Traceback (most recent call last):
            ...
        ValueError: list index out of range.
        >>> linked_list[len(linked_list)] = 666
        Traceback (most recent call last):
            ...
        ValueError: list index out of range.
        """
        if not 0 <= index < len(self):
            raise ValueError("list index out of range.")
        current = self.head
        for _ in range(index):
            current = current.next
        current.data = data

    def insert_tail(self, data: Any) -> None:
        """
        Insert data to the end of linked list.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("tail")
        >>> linked_list
        tail
        >>> linked_list.insert_tail("tail_2")
        >>> linked_list
        tail->tail_2
        >>> linked_list.insert_tail("tail_3")
        >>> linked_list
        tail->tail_2->tail_3
        """
        self.insert_nth(len(self), data)

    def insert_head(self, data: Any) -> None:
        """
        Insert data to the beginning of linked list.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_head("head")
        >>> linked_list
        head
        >>> linked_list.insert_head("head_2")
        >>> linked_list
        head_2->head
        >>> linked_list.insert_head("head_3")
        >>> linked_list
        head_3->head_2->head
        """
        self.insert_nth(0, data)

    def insert_nth(self, index: int, data: Any) -> None:
        """
        Insert data at given index.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        >>> linked_list.insert_nth(1, "fourth")
        >>> linked_list
        first->fourth->second->third
        >>> linked_list.insert_nth(3, "fifth")
        >>> linked_list
        first->fourth->second->fifth->third
        """
        if not 0 <= index <= len(self):
            raise IndexError("list index out of range")
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
        elif index == 0:
            new_node.next = self.head  # link new_node to head
            self.head = new_node
        else:
            temp = self.head
            for _ in range(index - 1):
                temp = temp.next
            new_node.next = temp.next
            temp.next = new_node

    def print_list(self) -> None:  # print every node data
        """
        This method prints every node data.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        """
        print(self)

    def delete_head(self) -> Any:
        """
        Delete the first node and return the
        node's data.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        >>> linked_list.delete_head()
        'first'
        >>> linked_list
        second->third
        >>> linked_list.delete_head()
        'second'
        >>> linked_list
        third
        >>> linked_list.delete_head()
        'third'
        >>> linked_list.delete_head()
        Traceback (most recent call last):
            ...
        IndexError: List index out of range.
        """
        return self.delete_nth(0)

    def delete_tail(self) -> Any:  # delete from tail
        """
        Delete the tail end node and return the
        node's data.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        >>> linked_list.delete_tail()
        'third'
        >>> linked_list
        first->second
        >>> linked_list.delete_tail()
        'second'
        >>> linked_list
        first
        >>> linked_list.delete_tail()
        'first'
        >>> linked_list.delete_tail()
        Traceback (most recent call last):
            ...
        IndexError: List index out of range.
        """
        return self.delete_nth(len(self) - 1)

    def delete_nth(self, index: int = 0) -> Any:
        """
        Delete node at given index and return the
        node's data.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        >>> linked_list.delete_nth(1) # delete middle
        'second'
        >>> linked_list
        first->third
        >>> linked_list.delete_nth(5) # this raises error
        Traceback (most recent call last):
            ...
        IndexError: List index out of range.
        >>> linked_list.delete_nth(-1) # this also raises error
        Traceback (most recent call last):
            ...
        IndexError: List index out of range.
        """
        if not 0 <= index <= len(self) - 1:  # test if index is valid
            raise IndexError("List index out of range.")
        delete_node = self.head  # default first node
        if index == 0:
            self.head = self.head.next
        else:
            temp = self.head
            for _ in range(index - 1):
                temp = temp.next
            delete_node = temp.next
            temp.next = temp.next.next
        return delete_node.data

    def is_empty(self) -> bool:
        """
        Check if linked list is empty.
        >>> linked_list = LinkedList()
        >>> linked_list.is_empty()
        True
        >>> linked_list.insert_head("first")
        >>> linked_list.is_empty()
        False
        """
        return self.head is None

    def reverse(self) -> None:
        """
        This reverses the linked list order.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first->second->third
        >>> linked_list.reverse()
        >>> linked_list
        third->second->first
        """
        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 test_singly_linked_list() -> None:
    """
    >>> test_singly_linked_list()
    """
    linked_list = LinkedList()
    assert linked_list.is_empty() is True
    assert str(linked_list) == ""

    try:
        linked_list.delete_head()
        raise AssertionError()  # This should not happen.
    except IndexError:
        assert True  # This should happen.

    try:
        linked_list.delete_tail()
        raise AssertionError()  # This should not happen.
    except IndexError:
        assert True  # This should happen.

    for i in range(10):
        assert len(linked_list) == i
        linked_list.insert_nth(i, i + 1)
    assert str(linked_list) == "->".join(str(i) for i in range(1, 11))

    linked_list.insert_head(0)
    linked_list.insert_tail(11)
    assert str(linked_list) == "->".join(str(i) for i in range(0, 12))

    assert linked_list.delete_head() == 0
    assert linked_list.delete_nth(9) == 10
    assert linked_list.delete_tail() == 11
    assert len(linked_list) == 9
    assert str(linked_list) == "->".join(str(i) for i in range(1, 10))

    assert all(linked_list[i] == i + 1 for i in range(0, 9)) is True

    for i in range(0, 9):
        linked_list[i] = -i
    assert all(linked_list[i] == -i for i in range(0, 9)) is True

    linked_list.reverse()
    assert str(linked_list) == "->".join(str(i) for i in range(-8, 1))


def test_singly_linked_list_2() -> None:
    """
    This section of the test used varying data types for input.
    >>> test_singly_linked_list_2()
    """
    test_input = [
        -9,
        100,
        Node(77345112),
        "dlrow olleH",
        7,
        5555,
        0,
        -192.55555,
        "Hello, world!",
        77.9,
        Node(10),
        None,
        None,
        12.20,
    ]
    linked_list = LinkedList()

    for i in test_input:
        linked_list.insert_tail(i)

    # Check if it's empty or not
    assert linked_list.is_empty() is False
    assert (
        str(linked_list) == "-9->100->Node(77345112)->dlrow olleH->7->5555->0->"
        "-192.55555->Hello, world!->77.9->Node(10)->None->None->12.2"
    )

    # Delete the head
    result = linked_list.delete_head()
    assert result == -9
    assert (
        str(linked_list) == "100->Node(77345112)->dlrow olleH->7->5555->0->-192.55555->"
        "Hello, world!->77.9->Node(10)->None->None->12.2"
    )

    # Delete the tail
    result = linked_list.delete_tail()
    assert result == 12.2
    assert (
        str(linked_list) == "100->Node(77345112)->dlrow olleH->7->5555->0->-192.55555->"
        "Hello, world!->77.9->Node(10)->None->None"
    )

    # Delete a node in specific location in linked list
    result = linked_list.delete_nth(10)
    assert result is None
    assert (
        str(linked_list) == "100->Node(77345112)->dlrow olleH->7->5555->0->-192.55555->"
        "Hello, world!->77.9->Node(10)->None"
    )

    # Add a Node instance to its head
    linked_list.insert_head(Node("Hello again, world!"))
    assert (
        str(linked_list)
        == "Node(Hello again, world!)->100->Node(77345112)->dlrow olleH->"
        "7->5555->0->-192.55555->Hello, world!->77.9->Node(10)->None"
    )

    # Add None to its tail
    linked_list.insert_tail(None)
    assert (
        str(linked_list)
        == "Node(Hello again, world!)->100->Node(77345112)->dlrow olleH->"
        "7->5555->0->-192.55555->Hello, world!->77.9->Node(10)->None->None"
    )

    # Reverse the linked list
    linked_list.reverse()
    assert (
        str(linked_list)
        == "None->None->Node(10)->77.9->Hello, world!->-192.55555->0->5555->"
        "7->dlrow olleH->Node(77345112)->100->Node(Hello again, world!)"
    )


def main():
    from doctest import testmod

    testmod()

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


if __name__ == "__main__":
    main()
About this Algorithm

Singly Linked List is a linear and connected data structure made of Nodes. Each node is composed of a variable data where its content is stored and a pointer to the next Node on the list. The Linked List has a pointer to the first element of this Node sequence and may also have another pointer to the last Node to make operations at the far end less time-consuming. You can also store a length variable to store the total length.

Advantages over Arrays

  • Size of a linked list is not fixed (dynamic size)
  • Deleting and adding an element is not expensive compared to an array

Drawbacks

  • Elements can be accessed sequentially not randomly compared to an array
  • Extra memory allocation needs to be done for pointers which connects elements in a linked list

Time Complexity

Operation Average Worst
Access O(n) O(n)
Search O(n) O(n)
Insertion O(1) O(1)
Deletion O(1) O(1)

Example

class LinkedList {
    Node head;      // Pointer to the first element
    Node tail;      // Optional. Points to the last element

    int length;     // Optional

    class Node {
        int data;   // Node data. Can be int, string, float, templates, etc
        Node next;  // Pointer to the next node on the list
    }
}

Code Implementation Links

Video Explanation

A CS50 video explaining the Linked List Data Structure