402\. Remove K Digits

Medium

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:**

*   <code>1 <= k <= num.length <= 10<sup>5</sup></code>
*   `num` consists of only digits.
*   `num` does not have any leading zeros except for the zero itself.