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5. Remove Element

easyAsked at CircleCI

Remove all occurrences of a target value from an array in-place.

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. The order of the remaining elements may be changed and you must return the number of elements not equal to val.

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 copy

Build a fresh array of non-val elements.

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

Tradeoff:

2. Two-pointer in-place

Use a write index that only advances for kept elements.

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];
  }
  return k;
}

Tradeoff:

CircleCI-specific tips

CircleCI uses in-place patterns when filtering finished job records out of an executor's work queue — show the constant-space version.

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Output

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