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5. Remove Element

easyAsked at Brex

Remove all occurrences of a value in-place from an array and return the new length.

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

Problem

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

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

Approaches

1. Brute force filter

Build a new array filtered by val and copy back.

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 pointers

Write pointer only advances on non-val. Single linear pass, no extra memory.

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:

Brex-specific tips

Brex interviewers like to follow this up with a question on filtering reversed or disputed transactions from a card-event array, so be ready to discuss why the relative order of survivors may or may not matter.

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