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5. Remove Element

easyAsked at Grab

Remove every occurrence of a value in-place — Grab uses this as a two-pointer drill.

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

Approaches

1. Filter copy

Build a new array of non-val elements.

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 tracks the write head; fast scans and writes only matching keeps.

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:

Grab-specific tips

Grab values candidates who note the in-place memory win when scaling to mobile clients — performance framing matters for a SEA-mobile-first stack.

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Output

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