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5. Remove Element

easyAsked at Mercury

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 an integer val, remove all occurrences of val in nums in-place and return the new length. The order of the elements may be changed.

Constraints

  • 0 <= nums.length <= 100
  • 0 <= nums[i], val <= 50

Examples

Example 1

Input
nums = [3,2,2,3], val = 3
Output
k=2, nums=[2,2,_,_]

Example 2

Input
nums = [0,1,2,2,3,0,4,2], val = 2
Output
k=5

Approaches

1. Filter and rewrite

Build a new array without val, copy back.

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

Tradeoff:

2. Two-pointer overwrite

Write pointer advances only when current value is kept; read pointer always advances. In-place and linear.

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:

Mercury-specific tips

At Mercury, this maps to filtering out cancelled wire instructions before sending the day's NACHA batch upstream, so emphasize that the originating-account index must remain stable post-filter.

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Output

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