How to Solve Palindrome Linked List on LeetCode
The Palindrome Linked List problem is a linked list challenge involving reversal and pointer techniques.
Problem Statement
Given the head of a singly linked list, return true if it is a palindrome.
Example:
Input: head = [1,2,2,1]
Output: true
Why This Problem Matters for Interviews
- Combines multiple linked list patterns (find middle, reverse, compare)
- Appears often as a medium-difficulty question
- Tests memory optimization and pointer skills
Approach to Palindrome Linked List
Find Middle, Reverse Second Half, Compare
- Use fast/slow pointers to find the list's middle.
- Reverse the second half.
- Compare both halves.
Time Complexity: O(n)
Space Complexity: O(1)
def isPalindrome(head):
# Find middle
slow = fast = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
# Reverse second half
prev = None
while slow:
slow.next, prev, slow = prev, slow, slow.next
# Compare halves
left, right = head, prev
while right:
if left.val != right.val:
return False
left, right = left.next, right.next
return True
Key Interview Talking Points
- Why O(1) space is possible (no extra array)
- Steps to reverse linked list in-place
- How to handle odd/even length lists
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Reverse Second Half, Compare | O(n) | O(1) |
Pro Interview Tips
- Draw lists for both odd and even cases
- Remember to restore the list if needed
- Explain all steps clearly