Remove Element Algorithm

Given an array and a target value, remove every occurrence of that value in-place. Return how many elements remain.

Order doesn't matter. Only the first k slots count.

Input: nums = [3,2,2,3], val = 3
Output: 2, nums = [2,2,,]

The intuition

Think of it like packing a suitcase. Walk through each item. If it's not the thing you want to remove, toss it into the "keep" pile. A write pointer tracks where that pile ends.

var removeElement = function(nums, val) {
    let writeIndex = 0;

    for (let i = 0; i < nums.length; i++) {
        if (nums[i] !== val) {
            nums[writeIndex] = nums[i];
            writeIndex++;
        }
    }

    return writeIndex;
};

Complexity

• Time: O(n) — single pass.
• Space: O(1) — no extra allocation.

When to use this

This is the simplest form of the two-pointer "partition" pattern. You'll see it everywhere: filtering arrays, compacting data, partitioning around a pivot in quicksort.

The cost of in-place: you lose the original array. If the caller needs it later, you've got a bug. Always know who owns the data.