O(N) Time

remove-duplicates.js

// O(N)

function remDupes(arr) {
  // have we seen this?
  let hash = [];

  // pointer 1
  p = -1;

  // establish i as pointer 2
  for (let i = 0; i < arr.length; i++) {
    console.log('arr at: ', i, arr);

    // if we haven't already seen it
    if (!hash[arr[i]]) {
      // add it
      hash[arr[i]] = arr[i];

      // move pointer 1
      if (p === -1) {
        p++;

      // if we made the spot undefined.
      } else if (arr[p] === undefined) {

        // pointer 1 location stores pointer 2 location.
        arr[p] = arr[i];

        // set pointer 2 location to undefined.
        arr[i] = undefined;

        // move pointer 1 up.
        p++;
      } else {

        // move pointer 1 up.
        p++;
      }

    // if we've seen it before.
    } else if (hash[arr[i]]) {

      // erase the duplicate
      arr[i] = undefined;

      // if the first pointer is not at an undefined, move it up.
      if (arr[p] !== undefined) {
        p++;
      }
    }
  }
  return arr;
}

console.log(
  'Expected [1, 2, 3, 5, 6, undefined, undefined, undefined]:', remDupes([
    1, 2, 1, 2, 3, 5, 5, 6
  ])
);