JavaScript Data Structures: Singly Linked List: Push

Intro

Last time, we setup our Singly Linked List.

Today, we learn how to push something to the list. Push means add something to the end.

Recap from last time

• we created a class Node
• we created a class Singly Linked List
• we learned how to create an instance of the Node class
• we learned how to create an instance of the Singly Linked List class

Current Code

// name of the class
class Node {
  // the constructor runs when using the class with `new` (see later)
  constructor(value) {
    // set this nodes value property to the instantiation value
    this.value = value;
    // set this nodes next property to `null`
    this.next = null;
  }
}

// name of the class
class SinglyLinkedList {
  // the constructor runs when using the class with `new`
  constructor() {
    // set this lists length property to `0`
    this.length = 0;
    // set this lists head property to `null`
    this.head = null;
    // set this lists tail property to `null`
    this.tail = null;
  }
}

Thoughts

First, we should think about the constraints and possibilities:

If there is already at least one other node in the Singly Linked List:

• create a new node with an input value
• point the current tails next property to the new node (so the new node comes after the current tail)
• set the Singly Linked List's tail to the new node
• increase the Singly Linked List's length by 1
• return the new node (so that we knew what we added)

If there is currently NO other node in the Singly Linked List (so it is currently empty):

• create a new node with an input value
• set the Singly Linked List's head to the new node
• set the Singly Linked List's tail to the new node
• increase the Singly Linked List's length by 1
• return the new node (so that we knew what we added)

The differences?

• if the Singly Linked List is empty, we need a head (the new node, because it is the only node)
• if the Singly Linked List already has at least one node, the last node should point to the new node and this new last node is the new tail

Example:

• 0 nodes: before: null (head & tail) => after: A (head & tail)
• 1 node: before: A (head & tail) => after: A (head) -> B (tail)
• 2 nodes: before: A (head) -> B (tail) => after: A (head) -> B -> C (tail)
• n nodes: before: A (head) -> ... -> n (tail) => after: A (head) -> ... -> n -> n+1 (tail)

So A always stays the head, only if we got 0 nodes, we have to set A as the new head.
In every other situation, we have to point the current tail to the new node and make the new node the new tail.

Implementation (Long version, no DRY)

• Comments from section Thoughts

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class SinglyLinkedList {
  constructor() {
    this.length = 0;
    this.head = null;
    this.tail = null;
  }

  push(value) {
    /*
     * 1. If there is already at least one other node in the Singly Linked List
     */
    if (this.length > 0) {
      // create a new node with an input value
      const newNode = new Node(value);
      // point the current tails `next` property to the new node
      this.tail.next = newNode;
      // set the Singly Linked List's `tail` to the new node
      this.tail = newNode;
      // increase the Singly Linked List's length by 1
      this.length += 1;
      // return the new node
      return newNode;
    } else {
      /*
       * 2. If there is currently NO other node in the Singly Linked List (so it is currently empty):
       */
      // create a new node with an input value
      const newNode = new Node(value);
      // set the Singly Linked List's `head` to the new node
      this.head = newNode;
      // set the Singly Linked List's `tail` to the new node
      this.tail = newNode;
      // increase the Singly Linked List's length by 1
      this.length += 1;
      // return the new node (so that we knew what we added)
      return newNode;
    }
  }
}

Implementation (Short version, DRY)

• we have a lot of duplicate code, because most of the logic is the same

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class SinglyLinkedList {
  constructor() {
    this.length = 0;
    this.head = null;
    this.tail = null;
  }

  push(value) {
    const newNode = new Node(value);
    if (this.length > 0) {
      this.tail.next = newNode;
    } else {
      this.head = newNode;
    }
    this.tail = newNode;
    this.length += 1;
    return newNode;
  }
}

Result

Let's have a look how to use the Singly Linked List push method and its results.

const newSLL = new SinglyLinkedList();
console.log(newSLL);
/*
 * SinglyLinkedList {
 *   length: 0,
 *   head: null,
 *   tail: null
 * }
 */

newSLL.push("A");
console.log(newSLL);
/*
 * SinglyLinkedList {
 *   length: 1,
 *   head: Node { value: 'A', next: null },
 *   tail: Node { value: 'A', next: null }
 * }
 */

newSLL.push("B");
console.log(newSLL);
/*
 * SinglyLinkedList {
 *   length: 2,
 *   head: Node { value: 'A', next: Node { value: 'B', next: null } },
 *   tail: Node { value: 'B', next: null }
 * }
 */

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Next Part

We will implement how to remove a node from the end of the Singly Linked List. If you want to be notified, subscribe :)