27. Remove Element

At a Glance

• Topic: arrays-hashing
• Pattern: Two Pointers
• Difficulty: Easy
• Companies: Amazon, Google, Apple, Microsoft, Bloomberg
• Frequency: High
• LeetCode: 27

Problem (one-liner)

Given integer array numbers and value remove, remove all instances of remove in-place with order preserved for remaining elements. Return the new length keptCount; first keptCount elements are the retained values in original relative order.

Recognition Cues

• "Remove all occurrences of value", "in-place", relative order
• Two pointers: writer for kept region, reader scans all

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 — shift left on each hit — O(n²) worst case.
• Better — count removals then second pass — two scans.
• Optimal — single pass swap-or-assign writer — O(n) time / O(1) space.

Optimal Solution

Go

package main

func removeElement(numbers []int, remove int) int {
	writeIndex := 0
	for scanIndex := 0; scanIndex < len(numbers); scanIndex++ {
		if numbers[scanIndex] != remove {
			numbers[writeIndex] = numbers[scanIndex]
			writeIndex++
		}
	}
	return writeIndex
}

JavaScript

function removeElement(numbers, remove) {
	let writeIndex = 0;
	for (let scanIndex = 0; scanIndex < numbers.length; scanIndex++) {
		if (numbers[scanIndex] !== remove) {
			numbers[writeIndex] = numbers[scanIndex];
			writeIndex++;
		}
	}
	return writeIndex;
}

Walkthrough

numbers = [3,2,2,3], remove = 3

Prefix [2,2], length 2.

Edge Cases

• No matches → return full length.
• All match → return 0.
• Empty array → 0.

Pitfalls

• Using extra array when in-place required.
• Confusing stable vs unstable removal when order not required (problem asks preserve order).

Similar Problems

• 026. Remove Duplicates from Sorted Array — conditional write.
• 283. Move Zeroes — partition around sentinel value.
• 088. Merge Sorted Array — pointer discipline.

Variants

• Remove duplicates unsorted — hash set or sort first.
• Remove range of indices.

Mind-Map Tags

#two-pointers #in-place #filter #arrays