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.

SOLUTION:

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        n = len(num)
        if n == k:
            return "0"
        stack = []
        for i in range(n):
            if len(stack) == 0:
                stack.append(num[i])
            else:
                while len(stack) > 0 and int(num[i]) < int(stack[-1]) and len(stack) >= i - k + 1:
                    stack.pop()
                if len(stack) == 0 or len(stack) < n - k:
                    stack.append(num[i])
        return str(int("".join(stack)))