O(1) storage

remove-duplicates2.js

// Storage O(1).

function removeDupes(arr) {
  // sort the array
  arr = arr.sort((a, b) => {
    return a - b;
  });

  // pointer 1
  p1 = 0;

  // pointer 2
  p2 = 1;

  while (p2 < arr.length) {
    console.log(`Array at p1:${p1}, p2:${p2}, array: ${arr}`);
    
    // if the two are the same, set the second to undefined.
    if (arr[p1] === arr[p2]) {
      arr[p2] = undefined;
      p2++;
    }

    // if the first is less than the second, move the first up.
    if (arr[p1] < arr[p2]) {
      p1++;
    }

    // if the first is undefined, trade with the second.
    if (arr[p1] === undefined) {
      arr[p1] = arr[p2];
      arr[p2] = undefined;

      // move the second up
      p2++;
    }
  }
  
  return arr;
}

console.log('Expected [1, 2, 3, 5, 6, undefined, undefined, undefined]:', '\n Actual [' + removeDupes([
  1, 2, 1, 2, 3, 5, 5, 6
]) + ']');