26. Array Problem -> Remove_Duplicates_from_Sorted_Array.md

Remove Duplicates from Sorted Array

📌 Problem

Given a sorted array nums, remove duplicates in-place such that each unique element appears only once.

Return the number of unique elements k.

• The first k elements of nums must contain the unique values
• Elements after index k - 1 do not matter

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📊 Constraints

• Array is sorted
• In-place modification (no extra array)

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💡 Core Idea

Use two pointers:

• One pointer reads elements
• One pointer writes unique elements

Instead of deleting duplicates, overwrite them.

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🔁 Algorithm

1. First element is always unique → set k = 1

2. Loop from index 1 to end

3. If current element ≠ previous element:

   • Write it at index k
   • Increment k

4. Return k

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🧠 Python Code (LeetCode Style)

class Solution:
    def removeDuplicates(self, nums):
        if not nums:
            return 0

        k = 1
        for i in range(1, len(nums)):
            if nums[i] != nums[i - 1]:
                nums[k] = nums[i]
                k += 1
        return k

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🔍 Example

Input

nums = [0,0,1,1,2,2,3]

Output

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

Only the first k elements are valid.

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⏱️ Time & Space

• Time: O(n)
• Space: O(1)

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🧩 Key Concepts Used

• Lists & indexing
• for loop
• if condition
• In-place modification
• Sorted array property

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🧠 Pattern Name

👉 Two Pointer Technique (Slow–Fast Pointer)

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