402. 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.

Complexity

• Time complexity: O(n)

• Space complexity: O(n)

Code

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        stack = []
        for i in num:
            while k > 0 and len(stack) > 0 and stack[-1] > i:
                k -= 1
                stack.pop()
            stack.append(i)
        if k > 0:
            stack = stack[:-k]

        return "".join(stack).lstrip("0") or "0"