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5. Remove Element

easyAsked at Slack

Remove all occurrences of val from an array in place and return the new length.

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

Problem

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. Return the number of elements that remain; the order of the kept elements may be changed.

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

Approaches

1. Filter copy

Filter array, then copy back.

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

Tradeoff:

2. Two pointers

Walk fast pointer; copy to slow pointer when value differs from val.

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

Tradeoff:

Slack-specific tips

Slack engineers will probe whether you preserve order; mention the swap-with-last optimization for unordered removals (mute-list cleanup).

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Output

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