Don't Miss Linked List Problems on LeetCode

1. Add Two Numbers

Problem: Given two non-empty linked lists representing two non-negative integers in reverse order, add the two numbers and return the sum as a linked list.

My Approach:

• Used a dummy node for simplified handling.

• Handled different lengths of input lists.

• Managed carry efficiently through each iteration.

Key Takeaway: Using a dummy node with a carry variable simplifies linked list math problems and avoids edge-case headaches.

2. Merge Two Sorted Lists

Problem: Merge two sorted linked lists into one sorted list.

My Approach:

• Compared nodes from both lists one by one.

• Initially created new nodes for the merged list.

• Later optimized the solution to reuse existing nodes instead of creating new ones — reducing memory usage.

Key Takeaway: Always check if you can solve linked list problems in-place by manipulating pointers instead of allocating memory.

3. Rotate Linked List

Problem: Rotate a linked list to the right by k places.

Initial Approach:

• Reversed list, extracted nodes, and re-reversed — which worked but was overly complex.

Optimized Approach:

• Converted the list to a circular list.

• Found the new head by stepping len - k steps.

• Broke the loop to form the rotated list.

Key Takeaway: For rotation-based problems, a circular pointer technique offers a clean and efficient in-place solution.

4. Swap Nodes in Pairs

Problem: Given a linked list, swap every two adjacent nodes and return its head.

My Approach:

• Initially created new nodes to simulate swapping.

• Optimized version used in-place swapping with pointer manipulation.

Key Takeaway: Most swapping problems can be solved with 3-pointer manipulation using a dummy node, keeping the space complexity O(1).

Lessons Learned

• Dummy nodes make code cleaner and easier to debug.

• Aim for in-place solutions unless the problem explicitly asks for deep copies.

• Practice and code review help in identifying inefficiencies early.

• Solving variations of the same data structure type helps deepen understanding.

What's Next?

Planning to take this further with:

• Reversing nodes in k-groups

• Detecting and removing cycles

• Flattening multilevel linked lists