Coding Challenge Practice - Question 105

Source: Dev.to

Problem

Create a function that removes duplicate values from an array in‑place (i.e., without allocating a new array).

Approach

1. Use a Set (seen) to keep track of values that have already been encountered.
2. Iterate through the array with a read pointer (readIndex).
3. Maintain a write pointer (writeIndex) that points to the position where the next unique element should be placed.
4. When an element has not been seen before:
   • Add it to seen.
   • Copy it to arr[writeIndex] (if writeIndex differs from readIndex).
   • Increment writeIndex.
5. After the loop, truncate the array to the new length (writeIndex).

Implementation

function deduplicate(arr) {
  const seen = new Set();
  let writeIndex = 0;

  for (let readIndex = 0; readIndex < arr.length; readIndex++) {
    const item = arr[readIndex];

    if (!seen.has(item)) {
      seen.add(item);

      if (writeIndex !== readIndex) {
        arr[writeIndex] = item;
      }
      writeIndex++;
    }
  }

  arr.length = writeIndex; // truncate to unique elements
  return arr;
}