Remove K Digits

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.