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"
Example 3:
Input: num = "10", k = 2 Output: "0"
Constraints:
1 <= k <= num.length <= 105num consists of only digits.num does not have any leading zeros except for the zero itself.