You are given a string num that holds a non-negative whole number, and a whole number k.
Remove exactly k digits from num so that the number left is as small as possible. The digits that stay keep their order. Return the smallest result as a string.
The result must not start with zeros, unless it is just "0". If you remove all the digits, the result is "0".
Examples
Example 1:
Input: num = "9112", k = 1
Output: "112"
Explanation: Remove the 9. The number 112 is the smallest possible.
Example 2:
Input: num = "30200", k = 1
Output: "200"
Explanation: Remove the 3. What is left is "0200", and after removing the zero at the front it is "200".
Example 3:
Input: num = "56", k = 2
Output: "0"
Explanation: All digits are removed, so the answer is "0".