LeetCode 80: Remove Duplicates from Sorted Array II — Python Solution

Solve LeetCode 80: Remove Duplicates from Sorted Array II in Python with a bounded write pointer 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

Allow a value when fewer than two values have been written or when it differs from the value two output positions back.

Algorithm

1. Allow a value when fewer than two values have been written or when it differs from the value two output positions back.
2. Maintain this invariant: No value occurs more than twice in the written prefix, and every permitted occurrence is retained.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

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

class Solution:
    def removeDuplicates(self, nums):
        write = 0
        for value in nums:
            if write < 2 or value != nums[write - 2]:
                nums[write] = value
                write += 1
        return write

Why this is correct

The proof follows the maintained state: No value occurs more than twice in the written prefix, and every permitted occurrence is retained. 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

Short arrays need no special rewriting; long duplicate runs collapse to two values.

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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