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5. Remove Element

easyAsked at Ola

Remove all occurrences of a value 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. The order of the elements may be changed. Return the number of elements in nums which 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

Approaches

1. Filter copy

Filter into a new array and copy back.

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

Write index advances only when the current value is not val. Single pass, no extra space.

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:

Ola-specific tips

Ola engineers favor the two-pointer write pattern when filtering tombstoned ride records from a list; emphasize stability and in-place mutation.

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