Question

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.
• The given num does not contain any leading zero.

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.

Solution

From left to right, remove digits that are larger than its right one. If less than k of such digits can be found (i.e. the sequence becomes nondecreasing), remove from right until k digits are removed. Remove any leading 0s.

from collections import deque
class Solution:
    def removeKdigits(self, num, k):
        """
        :type num: str
        :type k: int
        :rtype: str
        """
        if k <= 0:
            return num
        if len(num) == k:
            return "0"

        stack = deque()
        stack.append(num[0])

        for c in num[1:]:
            while k > 0 and stack and c < stack[-1]:
                stack.pop()
                k -= 1

            stack.append(c)

        while k > 0:
            stack.pop()
            k -= 1

        while stack and stack[0] == '0':
            stack.popleft()

        if not stack:
            stack.append('0')

        return ''.join(stack)