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Core Operations

Core Operations (Step by Step)

Section titled “Core Operations (Step by Step)”

Let’s build a Linked List from scratch — slowly and clearly.

🏗️ The Setup

Section titled “🏗️ The Setup”
// A single node = one box with value + arrow
class Node {
  constructor(val) {
    this.val = val;
    this.next = null;
  }
}


// The Linked List itself = just remembers the HEAD
class LinkedList {
  constructor() {
    this.head = null;  // empty list to start
  }
}

🔸 Insertion

Section titled “🔸 Insertion”

➕ Insert at the Beginning (Easy! O(1))

Section titled “➕ Insert at the Beginning (Easy! O(1))”

Goal: Add 5 to the start of 10 → 20 → 30.

flowchart LR
    subgraph Before[Before]
        H1[HEAD] --> N1[10] --> N2[20] --> N3[30] --> X1[NULL]
    end


    subgraph After[After]
        H2[HEAD] --> New[5 🆕] --> M1[10] --> M2[20] --> M3[30] --> X2[NULL]
    end


    style Before fill:#1e293b,color:#fff
    style After fill:#1e293b,color:#fff
    style New fill:#7c3aed,color:#fff
    style H2 fill:#f59e0b,color:#fff

Steps:

  1. Make a new node with value 5.
  2. Point new node’s next to current head (10).
  3. Update head to point to new node.
insertAtHead(val) {
  const node = new Node(val);  // Step 1: create node
  node.next = this.head;       // Step 2: point to old head
  this.head = node;            // Step 3: head is now the new node
}

➕ Insert at the End (O(n))

Section titled “➕ Insert at the End (O(n))”

Goal: Add 40 to end of 10 → 20 → 30.

flowchart LR
    H1[HEAD] --> A[10] --> B[20] --> C[30]
    C -->|walk to end| New[40 🆕] --> X1[NULL]


    style H1 fill:#f59e0b,color:#fff
    style C fill:#3b82f6,color:#fff
    style New fill:#7c3aed,color:#fff
    style X1 fill:#ef4444,color:#fff

Steps:

  1. Walk all the way to the last node.
  2. Make last node’s next point to the new node.
insertAtTail(val) {
  const node = new Node(val);
  if (!this.head) {              // empty list? new node IS the list
    this.head = node;
    return;
  }
  let curr = this.head;
  while (curr.next) {            // walk until last node
    curr = curr.next;
  }
  curr.next = node;              // attach new node
}

➕ Insert in the Middle

Section titled “➕ Insert in the Middle”

Goal: Insert 25 between 20 and 30.

flowchart LR
    subgraph Before[Before]
        H1[HEAD] --> A1[10] --> B1[20] --> C1[30] --> X1[NULL]
    end


    subgraph After[After]
        H2[HEAD] --> A2[10] --> B2[20] --> New[25 🆕] --> C2[30] --> X2[NULL]
    end


    B1 -.->|prev| B2


    style Before fill:#1e293b,color:#fff
    style After fill:#1e293b,color:#fff
    style New fill:#7c3aed,color:#fff
    style B2 fill:#3b82f6,color:#fff

Steps:

  1. Walk to the node just before where you want to insert (i.e., 20).
  2. Make new node point to prev.next (which is 30).
  3. Make prev.next point to the new node.
insertAt(val, idx) {
  if (idx === 0) return this.insertAtHead(val);


  const node = new Node(val);
  let prev = this.head;
  for (let i = 0; i < idx - 1; i++) prev = prev.next;


  node.next = prev.next;   // new node points to what prev pointed to
  prev.next = node;        // prev now points to new node
}

⚠️ The Golden Rule: Always set the new node’s next FIRST, then change prev.next. If you do it the other way, you lose the rest of the list!

🔸 Deletion

Section titled “🔸 Deletion”

➖ Delete from Beginning (Easy! O(1))

Section titled “➖ Delete from Beginning (Easy! O(1))”

Just move the head one step forward.

flowchart LR
    subgraph Before2[Before]
        H1[HEAD] --> D1[10 ❌] --> D2[20] --> D3[30]
    end


    subgraph After2[After]
        H2[HEAD] --> A2[20] --> A3[30]
    end


    style Before2 fill:#1e293b,color:#fff
    style After2 fill:#1e293b,color:#fff
    style D1 fill:#ef4444,color:#fff
    style H2 fill:#f59e0b,color:#fff
deleteHead() {
  if (!this.head) return;
  this.head = this.head.next;
}

➖ Delete a Specific Value

Section titled “➖ Delete a Specific Value”

Goal: Delete 20 from 10 → 20 → 30.

Steps:

  1. Find the node just before 20 (which is 10).
  2. Skip 20 by pointing 10.next directly to 30.
Before:  [10] ──▶ [20] ──▶ [30] ──▶ NULL
          prev    target


After:   [10] ──────────▶ [30] ──▶ NULL
                  ↑
              [20] is now orphaned (garbage collected)
deleteByValue(val) {
  if (!this.head) return;


  // Special case: deleting the head itself
  if (this.head.val === val) {
    this.head = this.head.next;
    return;
  }


  let curr = this.head;
  while (curr.next && curr.next.val !== val) {
    curr = curr.next;       // walk until next node has the target
  }


  if (curr.next) {
    curr.next = curr.next.next;  // skip the target
  }
}

🔸 Traversal (Walking the List)

Section titled “🔸 Traversal (Walking the List)”

Just like reading a treasure hunt, follow each clue until NULL.

print() {
  let curr = this.head;
  while (curr) {
    process.stdout.write(curr.val + " -> ");
    curr = curr.next;
  }
  console.log("NULL");
}
// Output: 10 -> 20 -> 30 -> NULL

🔸 Searching

Section titled “🔸 Searching”

Walk and check each node.

search(val) {
  let curr = this.head, idx = 0;
  while (curr) {
    if (curr.val === val) return idx;
    curr = curr.next;
    idx++;
  }
  return -1;  // not found
}
Section titled “Related”