LeetCode 27: Remove Element — Python Solution

Solve LeetCode 27: Remove Element in Python with a 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

Scan once and copy only values that should remain into the next output position. The prefix before the write pointer is the compacted result.

Algorithm

1. Scan once and copy only values that should remain into the next output position. The prefix before the write pointer is the compacted result.
2. Maintain this invariant: The prefix before write contains exactly the retained values seen so far.
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 removeElement(self, nums, val):
        write = 0
        for value in nums:
            if value != val:
                nums[write] = value
                write += 1
        return write

Why this is correct

The proof follows the maintained state: The prefix before write contains exactly the retained values seen so far. 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

All values may be removed, or none may match the target.

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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