LeetCode 92: Reverse Linked List II — Python Solution

Solve LeetCode 92: Reverse Linked List II in Python with a head insertion reversal approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Reach the node before the target segment, then repeatedly remove the next segment node and insert it at the segment front.

Algorithm

1. Reach the node before the target segment, then repeatedly remove the next segment node and insert it at the segment front.
2. Maintain this invariant: After each insertion, the processed portion of the target segment is reversed in place.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

LeetCode provides the list, tree, or graph node definition used by the method.

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def reverseBetween(self, head, left, right):
        dummy = ListNode(0, head)
        before = dummy
        for _ in range(left - 1):
            before = before.next
        tail = before.next
        for _ in range(right - left):
            moved = tail.next
            tail.next = moved.next
            moved.next = before.next
            before.next = moved
        return dummy.next

Why this is correct

The proof follows the maintained state: After each insertion, the processed portion of the target segment is reversed in place. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n) time and O(1) extra space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

Reversing from the head is handled by the dummy node; left may equal right.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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