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5. Remove Element

easyAsked at Instacart

Remove all occurrences of a value in place — Instacart uses this to test in-place array discipline before harder shopper inventory pruning problems.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an integer array nums and an integer val, remove all occurrences of val in place. Return the new length k after removal.

Constraints

  • 0 <= nums.length <= 100
  • 0 <= nums[i] <= 50
  • 0 <= val <= 100

Examples

Example 1

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

Example 2

Input
nums = [0,1,2,2,3,0,4,2], val = 2
Output
5, nums = [0,1,4,0,3,_,_,_]

Approaches

1. Filter and copy back

Create a filtered array, copy it into nums.

Time
O(n)
Space
O(n)
const keep = nums.filter(x => x !== val);
for (let i = 0; i < keep.length; i++) nums[i] = keep[i];
return keep.length;

Tradeoff:

2. Two pointers

Slow pointer marks the next keep slot; skip matches.

Time
O(n)
Space
O(1)
function removeElement(nums, val) {
  let k = 0;
  for (let i = 0; i < nums.length; i++) {
    if (nums[i] !== val) {
      nums[k] = nums[i];
      k++;
    }
  }
  return k;
}

Tradeoff:

Instacart-specific tips

Instacart wants you to skip the auxiliary allocation — they'll ask how this generalizes to removing out-of-stock items from an active shopper cart.

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Output

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