Merge Two Sorted Linked Lists MASTERCLASS | Iterative, Recursive, K-Lists

Описание: Merging Linked Lists is one of the most important interview patterns — especially Merge Two Sorted Lists.

In this video, we go from Beginner → Advanced and cover everything you need for coding interviews.

🟢 Level 1: Understand the Problem

Most interview questions mean:

👉 Merge two sorted linked lists

Example:

List A:
1 → 3 → 5

List B:
2 → 4 → 6

Result:
1 → 2 → 3 → 4 → 5 → 6

Think of it like merging two sorted arrays — but with pointers.

🟢 Level 2: Core Idea (Very Important)

We:

✔ Compare values
✔ Attach the smaller one
✔ Move that pointer
✔ Repeat

Simple idea. Powerful pattern.

🟢 Level 3: Iterative Solution (Most Important)

We use a dummy node (very important trick).

Why dummy?
It simplifies head handling and avoids edge cases.

Complexity:

Time: O(n + m)
Space: O(1)

🔥 Interview Question:
Why do we return dummy.next instead of dummy?

🟡 Level 4: Recursive Merge

Cleaner and elegant.

But:

Space: O(n + m) (due to recursion stack)

Used when interviewer wants clean thinking.

🟠 Level 5: Merge Two Unsorted Lists

Two approaches:

1️⃣ Just attach second list at end (if order doesn’t matter)
2️⃣ If sorted result required → combine then sort

Best method: Use Merge Sort

🔵 Level 6: Merge K Sorted Lists (Advanced)

Example:

List1: 1 → 4 → 7
List2: 2 → 5 → 8
List3: 3 → 6 → 9

Efficient approaches:

1️⃣ Min-Heap → O(N log k)
2️⃣ Divide & Conquer → O(N log k)

Very common FAANG interview question.

🟣 Level 7: Merge Sort on Linked List

Merge is the backbone of Merge Sort.

Steps:

✔ Find middle (fast & slow pointer)
✔ Split list
✔ Recursively sort
✔ Merge

Time: O(n log n)
Space: O(log n)

🧠 Advanced Variations

✔ Merge in reverse order
✔ Merge alternating nodes
✔ Merge and remove duplicates
✔ Merge circular linked lists
✔ Merge doubly linked lists

⚠️ Common Mistakes

❌ Forgetting dummy node
❌ Losing next pointer
❌ Not attaching remaining list
❌ Returning wrong head
❌ Modifying original head incorrectly

🎯 Mini Challenge

If:

L1:
1 → 2 → 4

L2:
1 → 3 → 4

What’s the merged result?

Pause the video and try it before checking.
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