Remove K Digits

Problem Statement

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

Leetcode link

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.

Code

Python Code

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        st = list()
        for n in num:
            while st and k and st[-1] > n:
                st.pop()
                k -= 1

            if st or n is not '0': # prevent leading zeros
                st.append(n)

        if k: # not fully spent
            st = st[0:-k]

        return ''.join(st) or '0'