26. Remove Duplicates from Sorted Array

At a Glance

• Topic: arrays-hashing
• Pattern: Two Pointers
• Difficulty: Easy
• Companies: Amazon, Google, Meta, Microsoft, Bloomberg
• Frequency: High
• LeetCode: 26

Problem (one-liner)

Given sorted integers numbers, remove duplicates in-place so each unique value appears once. Return uniqueCount; first uniqueCount slots contain sorted unique values in order.

Recognition Cues

• "Sorted array", "remove duplicates in-place"
• Slow pointer for write position, fast for scan

Diagram

At-a-glance flow (replace with problem-specific Mermaid as you refine this note). camelCase node IDs; no spaces in IDs.

Approaches

• Brute force — new array or hash set — O(n) space extra.
• Better — same two-pointer but confusing variable names.
• Optimal — writeIndex starts at 1; copy forward when numbers[scan] != numbers[writeIndex-1] — O(n) time / O(1) extra space.

Optimal Solution

Go

package main

func removeDuplicates(numbers []int) int {
	if len(numbers) == 0 {
		return 0
	}
	writeIndex := 1
	for scanIndex := 1; scanIndex < len(numbers); scanIndex++ {
		if numbers[scanIndex] != numbers[writeIndex-1] {
			numbers[writeIndex] = numbers[scanIndex]
			writeIndex++
		}
	}
	return writeIndex
}

JavaScript

function removeDuplicates(numbers) {
	if (numbers.length === 0) {
		return 0;
	}
	let writeIndex = 1;
	for (let scanIndex = 1; scanIndex < numbers.length; scanIndex++) {
		if (numbers[scanIndex] !== numbers[writeIndex - 1]) {
			numbers[writeIndex] = numbers[scanIndex];
			writeIndex++;
		}
	}
	return writeIndex;
}

Walkthrough

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

Final prefix [0,1,2,3,4,...] length 5.

Edge Cases

• Empty array → 0.
• All distinct → return len(numbers).
• Single element → 1.

Pitfalls

• Returning new slice instead of mutating in-place when required.
• Comparing with wrong prior element after compaction.

Similar Problems

• 027. Remove Element — same write-pointer pattern.
• 088. Merge Sorted Array — ordered merge discipline.
• 283. Move Zeroes — partition-style writes.

Variants

• Allow at most k duplicates (LeetCode 80).
• Linked list dedupe.

Mind-Map Tags

#two-pointers #sorted #dedupe #in-place