LeetCode 26: Remove Duplicates from Sorted Array — Python Solution

Solve LeetCode 26: Remove Duplicates from Sorted Array in Python with a sorted 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

Keep one write position for the next distinct value. Because the input is sorted, comparing with the last written value is sufficient.

Algorithm

1. Keep one write position for the next distinct value. Because the input is sorted, comparing with the last written value is sufficient.
2. Maintain this invariant: The written prefix contains one copy of every distinct value in sorted order.
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):
        if not nums:
            return 0
        write = 1
        for read in range(1, len(nums)):
            if nums[read] != nums[write - 1]:
                nums[write] = nums[read]
                write += 1
        return write

Why this is correct

The proof follows the maintained state: The written prefix contains one copy of every distinct value in sorted order. 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

Handle an empty array and arrays containing only one repeated value.

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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