What is the time and space complexity of Remove Duplicates from Sorted Array II?

Time complexity: O(n). Space complexity: O(1).

Remove Duplicates from Sorted Array II

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Solution Guide: Remove Duplicates from Sorted Array II

Approach

Use a slow pointer starting at index 2. The fast pointer scans from index 2 onward. Compare nums[fast] with nums[slow - 2]: if they differ, the element is safe to keep (at most 2 copies), so copy it to slow and advance slow.

Complexity Analysis

Time: O(n)
Space: O(1)

Pattern

Two Pointers (Same Direction)

Why It Works

Comparing with nums[slow - 2] ensures at most two copies of any value survive. If nums[fast] equals nums[slow - 2], there are already two copies in the valid region, so it must be skipped.

Updated Feb 2026