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5. Remove Element

easyAsked at Duolingo

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

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

Problem

Given an integer array nums and a target val, remove all occurrences of val from nums in-place. Return the new length k; the first k positions must contain the surviving elements in any order.

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. Splice loop

Walk the array and splice each matching element.

Time
O(n^2)
Space
O(1)
for (let i = nums.length - 1; i >= 0; i--)
  if (nums[i] === val) nums.splice(i, 1);
return nums.length;

Tradeoff:

2. Two pointers

Keep a write index; copy any non-val element forward and increment it.

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:

Duolingo-specific tips

Duolingo's lesson editor frequently strips placeholder tokens from exercise arrays; the two-pointer compaction is the canonical Duolingo bulk-filter pattern.

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