How to Solve Reverse Linked List on LeetCode
The Reverse Linked List problem is a classic data structures question that tests your mastery of pointer manipulation, iteration, and recursion.
Problem Statement
Given the head of a singly linked list, reverse the list, and return the reversed list.
Example:
Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]
Why This Problem Matters for Interviews
This problem:
- Assesses pointer manipulation and understanding of linked list nodes
- Distinguishes between iterative and recursive approaches
- Forms a base for more complex linked list operations
Approaches to Reverse Linked List
1. Iterative Approach (Optimal)
Use three pointers: previous, current, next.
Time Complexity: O(n)
Space Complexity: O(1)
def reverseList(head):
prev = None
curr = head
while curr:
nxt = curr.next
curr.next = prev
prev = curr
curr = nxt
return prev
2. Recursive Approach
Reverse pointers as you return from recursion.
Time Complexity: O(n)
Space Complexity: O(n) for recursion stack
def reverseList(head):
if not head or not head.next:
return head
p = reverseList(head.next)
head.next.next = head
head.next = None
return p
Key Interview Talking Points
- Compare iterative and recursive trade-offs
- Discuss space complexity of recursion
- Show how reversing helps in other problems (add two numbers, etc.)
- Trace pointer updates step-by-step
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Iterative | O(n) | O(1) |
| Recursive | O(n) | O(n) |
Pro Interview Tips
- Walk through pointer updates on sample inputs.
- Mention base cases and why they're needed in recursion.
- Practice with test cases