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402. Remove K Digits

String Stack Greedy Monotonic Stack

Problem - 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.

Solutions

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class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        stack = []
        remaining = len(num) - k
        for char in num:
            while k and stack and stack[-1] > char:
                stack.pop()
                k -= 1
            stack.append(char)
        return "".join(stack[:remaining]).lstrip("0") or "0"

Submission Stats:

  • Runtime: 20 ms (57.18%)
  • Memory: 19 MB (41.53%)