402. Remove K Digits

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.

class Solution(object):
    def removeKdigits(self, num, k):
        """
        :type num: str
        :type k: int
        :rtype: str
        """
        if k == len(num):
            return "0"
        s = []
        for c in num:
            if s == []:
                s.append(c)
            else:
                while s and c < s[-1] and k > 0:
                    s.pop()
                    k -= 1
                else:
                    s.append(c)
        
        while k > 0:
            s.pop()
            k -= 1
        total = 0
        for c in s:
            total = total*10 + int(c)
        
        return str(total)