LeetCode 82: Remove Duplicates from Sorted List II — Python Solution

Solve LeetCode 82: Remove Duplicates from Sorted List II in Python with a run skipping 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

Use a dummy predecessor. When equal adjacent values begin a duplicate run, skip the entire run; otherwise advance normally.

Algorithm

1. Use a dummy predecessor. When equal adjacent values begin a duplicate run, skip the entire run; otherwise advance normally.
2. Maintain this invariant: The list before prev contains only values that appeared exactly once in processed input.
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 deleteDuplicates(self, head):
        dummy = ListNode(0, head)
        previous = dummy
        while head:
            if head.next and head.val == head.next.val:
                duplicate = head.val
                while head and head.val == duplicate:
                    head = head.next
                previous.next = head
            else:
                previous, head = head, head.next
        return dummy.next

Why this is correct

The proof follows the maintained state: The list before prev contains only values that appeared exactly once in processed input. 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

Duplicate runs may appear at the head or tail.

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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