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When working with sorted arrays, one common interview problem is removing duplicates in-place while maintaining the relative order of elements. Let’s break down LeetCode 26: Remove Duplicates from Sorted Array and walk through an efficient solution in JavaScript.

🚀 Problem Description

Given an integer array nums sorted in non-decreasing order, remove duplicates in-place so that each unique element appears only once. Return the count of unique elements (k) and modify the array so the first k elements contain the unique values.
The rest of the array doesn't matter.

💡 Examples

Example 1

Input: nums = [1,1,2]
Output: 2, nums = [1,2,_]

Example 2

Input: nums = [0,0,1,1,1,2,2,3,3,4]
Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]

🧠 Key Insights

• Sorted input: Since the array is sorted, duplicates will always appear consecutively.
• Two-pointer technique: Use two pointers to traverse and overwrite the array while identifying unique elements.

🏆 JavaScript Solution: Two-Pointer Approach

Here’s the implementation:

var removeDuplicates = function(nums) {
    if (nums.length === 0) return 0; // Handle edge case

    let k = 1; // Pointer for the next unique element position

    for (let i = 1; i < nums.length; i++) {
        if (nums[i] !== nums[i - 1]) {
            nums[k] = nums[i];
            k++;
        }
    }

    return k; // Count of unique elements
};

🔍 How It Works

1. Initialize pointers:
   • Start k at 1 to represent the position of the next unique element.
2. Iterate through the array:
   • If nums[i] is different from the previous element, it’s unique. Copy it to nums[k] and increment k.
3. Return k:
   • This is the count of unique elements. The first k elements in nums now hold these values.

🔑 Complexity Analysis

• > Time Complexity: O(n), where n is the length of the array. We traverse the array once.
• > Space Complexity: O(1), since no extra space is used.

📋 Dry Run

Input: nums = [0,0,1,1,1,2,2,3,3,4]

Output: k = 5, nums = [0,1,2,3,4,_,_,_,_,_]

✨ Pro Tips for Interviews

1. Ask clarifying questions:
   • Confirm if the order of elements must be preserved.
2. Consider edge cases:
   • Empty array.
   • Array with all identical elements.
3. Explain the two-pointer logic clearly:
   • It’s simple but highly effective for problems requiring in-place modifications.

📚 Learn More

Check out the full explanation and code walkthrough on Dev.to:
👉 Remove Element - JavaScript Solution

How would you approach this problem? Let me know in the comments! 🚀

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