27.Remove Element

📘 LeetCode 27: Remove Element

📌 Problem Overview

• Problem Number: 27
• Problem Name: Remove Element
• Difficulty: Easy
• Category: Two Pointers
• Runnable Source: Program.cs

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💡 Algorithmic Approach & Pattern

This problem is solved using the Two Pointers algorithmic pattern.

Key Insights & Invariants:

1. Identify the input constraints and edge conditions.
2. Structure the data flow to minimize unnecessary traversals and heap reallocations.
3. Validate corner cases (empty collections, single-element cases, duplicates, extremes).

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💻 Verified C# Implementation (.NET 9)

The solution is verified and executable via the companion project in src/27.remove-element/:

System.Console.WriteLine(RemoveElement([3, 2, 2, 3], 3));
System.Console.WriteLine(RemoveElement([0, 1, 2, 2, 3, 0, 4, 2], 2));
 
int RemoveElement(int[] nums, int val)
{
    int index = 0;
    for (int i = 0; i < nums.Length; i++)
    {
        if (nums[i] != val)
        {
            nums[index] = nums[i];
            index++;
        }
    }
    return index;
    // nums = [.. nums.Where(x => x != val).Select(x => x)];
    // return nums.Length;
}

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📊 Complexity Analysis

Metric	Complexity	Notes
Time Complexity	$\mathcal{O}(N)$ (or optimal for pattern)	Single or bounded passes over the input
Space Complexity	$\mathcal{O}(1)$ or $\mathcal{O}(N)$	Minimal auxiliary memory overhead

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🚨 Edge Cases & Interview Traps

• Empty / Null Input: Ensure null safety and guard clauses.
• Single Element: Confirm the algorithm terminates without out-of-bounds indexing.
• Duplicates & Boundaries: Check duplicate handling and extreme integer limits.