Leetcode #92: Reverse Linked List II
In this guide, we solve Leetcode #92 Reverse Linked List II in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right, and return the reversed list. Example 1: Input: head = [1,2,3,4,5], left = 2, right = 4 Output: [1,4,3,2,5] Example 2: Input: head = [5], left = 1, right = 1 Output: [5] Constraints: The number of nodes in the list is n.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Linked List
Intuition
Linked list problems often require pointer manipulation rather than extra memory.
Two-pointer techniques expose cycles, midpoints, or reordering patterns.
Approach
Traverse with fast/slow pointers or reverse sublists when needed.
Maintain invariants carefully to avoid losing nodes.
Steps:
- Traverse with pointers.
- Reverse or split if required.
- Reconnect nodes correctly.
Example
Input: head = [1,2,3,4,5], left = 2, right = 4
Output: [1,4,3,2,5]
Python Solution
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def reverseBetween(
self, head: Optional[ListNode], left: int, right: int
) -> Optional[ListNode]:
if head.next is None or left == right:
return head
dummy = ListNode(0, head)
pre = dummy
for _ in range(left - 1):
pre = pre.next
p, q = pre, pre.next
cur = q
for _ in range(right - left + 1):
t = cur.next
cur.next = pre
pre, cur = cur, t
p.next = pre
q.next = cur
return dummy.next
Complexity
The time complexity is , and the space complexity is . The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.