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:

• 1 <= k <= num.length <= 105

• num consists of only digits.

• num does not have any leading zeros except for the zero itself.

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        stack = []
        for n in map(int, list(num)):
            while stack and k > 0 and stack[-1] > n:
                stack.pop()
                k -= 1
            stack.append(n)
        while k > 0 and stack:
            stack.pop()
            k -= 1
        if not stack:
            return '0'
        num = ''.join(map(str, stack))
        index = 0
        ptr = 0
        while index < len(num) and num[index] == '0':
            ptr += 1
            index += 1
        return "0" if ptr == len(num) else num[ptr:]