Given an array of integers `arr` and an integer `k`. Find the least number of unique integers after removing exactly `k` elements.

Example 1:

    Input: arr = [5,5,4], k = 1

    Output: 1

    Explanation: Remove the single 4, only 5 is left.

Example 2:

    Input: arr = [4,3,1,1,3,3,2], k = 3

    Output: 2

    Explanation: Remove 4, 2 and either one of the two 1s or three 3s. 1 and 3 will be left.

Constraints:

- `1 <= arr.length <= 10^5`
- `1 <= arr[i] <= 10^9`
- `0 <= k <= arr.length`