Introduction

Reverse Linked List II means reversing only a specific portion of a linked list.

The task is to:

  • reverse nodes from left to right position
  • keep remaining nodes unchanged
  • reconnect reversed portion correctly

Example:

Input:1 -> 2 -> 3 -> 4 -> 5
left = 2
right = 4

Output:
1 -> 4 -> 3 -> 2 -> 5

Explanation:

Nodes between 2 and 4 are reversed.Remaining nodes stay unchanged.

This problem is one of the most important applications of:

Pointer Manipulation

Constraints

1 <= Number of Nodes <= 10^5
1 <= left <= right <= n

Approach 1 : Brute Force (Using Array)

Explanations:

Explanation:

The idea is:

  • store linked list values in array
  • reverse subarray
  • rebuild linked list

Steps:

  1. Traverse linked list.
  2. Store node values.
  3. Reverse selected portion.
  4. Create updated linked list.

This approach works but:

  • uses extra memory

So in-place reversal is preferred.

Dry Run

Input:1 -> 2 -> 3 -> 4 -> 5
left = 2
right = 4

Array:
[1,2,3,4,5]

Reverse:
[2,3,4]

Updated:
[1,4,3,2,5]

Output:
1 -> 4 -> 3 -> 2 -> 5

Practice :

Complexity Analysis :

Time Complexity:- O(n)Explanation :
Linked list traversal takes linear time.

Space Complexity:- O(n)
Explanation :
Extra array is used.

Approach 2 : Optimal Solution(Using Pointer Reversal)

Explanations:

Explanation:

This is the most optimized and interview-preferred solution.

The idea is:

  • reach left position
  • reverse links until right position
  • reconnect remaining nodes

This performs reversal in-place.

Dry Run

Input:1 -> 2 -> 3 -> 4 -> 5

left = 2
right = 4

Reverse:
2 -> 3 -> 4

Becomes:
4 -> 3 -> 2

Reconnect:
1 -> 4 -> 3 -> 2 -> 5

Practice :

Complexity Analysis :

Time Complexity:- O(n)Explanation :
Linked list is traversed once.

Space Complexity:- O(1)
Explanation :
No extra space is used.

Why This Problem is Important

This problem builds the foundation for:

  • Pointer manipulation
  • Sublist reversal
  • In-place algorithms
  • Linked List traversal
  • Memory optimization

Real-World Applications

Partial reversal concepts are used in:

  • Data transformations
  • Text processing systems
  • Playlist modifications
  • Undo/Redo systems
  • Dynamic memory operations

Common Beginner Mistakes

  • Incorrect reconnection logic
  • Losing next node reference
  • Wrong left/right indexing
  • Infinite loops
  • Breaking linked structure

Interview Tip

Interviewers often expect:

  • in-place reversal logic
  • proper reconnection handling
  • O(1) space optimization
  • clean pointer manipulation

Always explain:

  • how sublist is isolated
  • how reversed part reconnects safely

Related Questions

  • Reverse Linked List
  • Reverse Nodes in K Group
  • Palindrome Linked List
  • Linked List Cycle
  • Merge Two Lists

Final Takeaway

The Reverse Linked List II problem is one of the most important linked list reversal problems.

It teaches:

  • partial reversal
  • pointer manipulation
  • sublist handling
  • in-place algorithms

Understanding this problem builds a strong foundation for:

  • advanced linked list problems
  • pointer-based algorithms
  • interview-level reversal questions.