Remove Duplicates from Sorted Array

Collect all unique elements into a separate array, then copy them back. This is simple but uses extra space.

1. 1Iterate through nums and collect unique elements into a list.
2. 2Copy them back into nums in order.
3. 3Return the count of unique elements.

Why it's not optimal: Uses O(n) extra space. The two-pointer approach does it truly in-place with O(1) space.

def removeDuplicates(nums):
    unique = []
    for num in nums:
        if not unique or unique[-1] != num:
            unique.append(num)
    for i in range(len(unique)):
        nums[i] = unique[i]
    return len(unique)

Use a slow pointer k to track the write position (next slot for a unique element) and a fast pointer i to scan the array. Whenever nums[i] differs from the last unique value written, copy it to position k.

1. 1Start k=1 (first element is always unique) and i=1.
2. 2If nums[i] != nums[k-1], write nums[k] = nums[i] and increment k.
3. 3Always advance i each iteration. Duplicates are simply skipped.
4. 4Return k — the count of unique elements.