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5. Remove Element

easyAsked at Coursera

Remove all instances of a value in-place — Coursera uses this to test two-pointer fluency for filtering enrollment lists.

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

Problem

Given an array nums and integer val, remove all occurrences of val in-place. Return the new length; order of remaining elements may change.

Constraints

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

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 = [0,1,3,0,4,_,_,_]

Approaches

1. Filter and copy back

Filter to a new array, copy back.

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

Tradeoff:

2. Two pointers

Write pointer advances only on keep; iterate read pointer once.

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:

Coursera-specific tips

Coursera reviewers map this to filtering unenrolled learners from a roster array in-place — say it out loud so they see you connect the abstract pattern to a real data ops chore.

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