Using an array and moving duplicates to end

Question

I got this question at an interview and at the end was told there was a more efficient way to do this but have still not been able to figure it out. You are passing into a function an array of integers and an integer for size of array. In the array you have a lot of numbers, some that repeat for example 1,7,4,8,2,6,8,3,7,9,10. You want to take that array and return an array where all the repeated numbers are put at the end of the array so the above array would turn into 1,7,4,8,2,6,3,9,10,8,7. The numbers I used are not important and I could not use a buffer array. I was going to use a BST, but the order of the numbers must be maintained(except for the duplicate numbers). I could not figure out how to use a hash table so I ended up using a double for loop(n^2 horrible I know). How would I do this more efficiently using c++. Not looking for code, just an idea of how to do it better.

Answer 1

In what follows:

1. arr is the input array;
2. seen is a hash set of numbers already encountered;
3. l is the index where the next unique element will be placed;
4. r is the index of the next element to be considered.

Since you're not looking for code, here is a pseudo-code solution (which happens to be valid Python):

arr = [1,7,4,8,2,6,8,3,7,9,10]
seen = set()
l = 0
r = 0
while True:
  # advance `r` to the next not-yet-seen number
  while r < len(arr) and arr[r] in seen:
    r += 1
  if r == len(arr): break
  # add the number to the set
  seen.add(arr[r])
  # swap arr[l] with arr[r]
  arr[l], arr[r] = arr[r], arr[l]
  # advance `l`
  l += 1
print arr

On your test case, this produces

[1, 7, 4, 8, 2, 6, 3, 9, 10, 8, 7]

Answer 2

I would use an additional map, where the key is the integer value from the array and the value is an integer set to 0 in the beginning. Now I would go through the array and increase the values in the map if the key is already in the map. In the end I would go again through the array. When the integer from the array has a value of one in the map, I would not change anything. When it has a value of 2 or more in the map I would swap the integer from the array with the last one.

This should result in a runtime of O(n*log(n))