Basics

Design

707. Design Linked List

Medium2

HackerRank Linked List BasicsEasy

707. Design Linked List

Single Linked List

Interactive Visualization

Python

class SinglyLLNode:
    def __init__(self, val: Any, next=None):
        self.val = val
        self.next = next


class MyLinkedList:
    def __init__(self):
        self.sentinel = SinglyLLNode(None, None)
        self.len = 0

    def get(self, index: int) -> int:
        node = self.goToIndex(index)
        return node.val if node else -1

    def addAtHead(self, val: int) -> None:
        new_node = SinglyLLNode(val)
        new_node.next = self.sentinel.next
        self.sentinel.next = new_node
        self.len += 1

    def addAtTail(self, val: int) -> None:
        new_node = SinglyLLNode(val)
        current_node = self.sentinel
        while current_node.next:
            current_node = current_node.next
        current_node.next = new_node
        self.len += 1

    def addAtIndex(self, index: int, val: int) -> None:
        prev_node = self.goToIndex(index - 1)
        if prev_node:
            new_node = SinglyLLNode(val)
            new_node.next = prev_node.next
            prev_node.next = new_node
            self.len += 1

    def goToIndex(self, index: int) -> Optional[SinglyLLNode]:
        current_index = -1
        current_node = self.sentinel
        while current_node.next and current_index < index:
            current_node = current_node.next
            current_index += 1
        if current_index == index:
            return current_node
        return None

    def deleteAtIndex(self, index: int) -> None:
        prev_node = self.goToIndex(index - 1)
        if prev_node and prev_node.next:
            prev_node.next = prev_node.next.next
        self.len -= 1

Doubly Linked List

Interactive Visualization

Python

class DoublyLLNode:
    def __init__(self, val: Any, prev=None, next=None):
        self.val = val
        self.prev = prev
        self.next = next


class MyLinkedList:
    def __init__(self):
        self.sentinel = DoublyLLNode(None, None, None)
        self.tail = DoublyLLNode(None, self.sentinel, None)
        self.sentinel.next = self.tail
        self.len = 0

    def goToIndex(self, index: int) -> Optional[DoublyLLNode]:
        current_index = -1
        current_node = self.sentinel
        while current_node.next != self.tail and current_index < index:
            current_node = current_node.next
            current_index += 1
        if current_index == index:
            return current_node
        return None

    def get(self, index: int) -> int:
        node = self.goToIndex(index)
        self.print()
        return node.val if node else -1

    def addAtHead(self, val: int) -> None:
        new_node = DoublyLLNode(val, prev=self.sentinel, next=self.sentinel.next)
        self.sentinel.next = new_node
        new_node.next.prev = new_node
        self.len += 1

    def addAtTail(self, val: int) -> None:
        new_node = DoublyLLNode(val, prev=self.tail.prev, next=self.tail)
        self.tail.prev = new_node
        new_node.prev.next = new_node
        self.len += 1

    def addAtIndex(self, index: int, val: int) -> None:
        prev_node = self.goToIndex(index - 1)
        if prev_node:
            new_node = DoublyLLNode(val, prev=prev_node, next=prev_node.next)
            prev_node.next = new_node
            new_node.next.prev = new_node
            self.len += 1

    def deleteAtIndex(self, index: int) -> None:
        if not 0 <= index < self.len:
            return
        prev_node = self.goToIndex(index - 1)
        prev_node.next.next.prev = prev_node
        prev_node.next = prev_node.next.next
        self.len -= 1

    def print(self):
        cur = self.sentinel.next
        print("HEAD -> ", end="")
        while cur and cur.val:
            print(f"{cur.val} -> ", end="")
            cur = cur.next
        print("TAIL")
        cur = self.tail.prev
        print("TAIL -> ", end="")
        while cur and cur.val:
            print(f"{cur.val} -> ", end="")
            cur = cur.prev
        print("HEAD")

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Reusable Code

Count Linked List Nodes

Basic

Count Linked List Nodes

def getLen(head):
    length = 0
    while head:
        head = head.next
        length += 1
    return length

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Print the Elements of a Linked List

Easy

Print the Elements of a Linked List

def print_ll(head, attr_name="val"):
    while head:
        value = getattr(head, attr_name, None)
        print(value, end=" -> " if head.next else "\n")
        head = head.next

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