Given string num representing a non-negative integer `num`, and an integer `k`, return the smallest possible integer after removing `k` digits from `num`.

Example 1:

    Input: num = "1432219", k = 3
    Output: "1219"
    Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

    Input: num = "10200", k = 1
    Output: "200"
    Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

    Input: num = "10", k = 2
    Output: "0"
    Explanation: Remove all the digits from the number and it is left with nothing which is 0.

Constraints:

- `1 <= k <= num.length <= 105`
- `num` consists of only digits.
- `num` does not have any leading zeros except for the zero itself.