Remove K Digits.py

'''
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
        """
        vec = []
        for c in num:
            while vec and k and vec[-1] > c:
                vec.pop()
                k -= 1
            vec.append(c)
        
        while vec and k:
            vec.pop()
            k -= 1
            
        res = ''.join(vec)
        while len(res) > 0 and res[0] == '0':
            res = res[1:]
        if res:
            return res
        else:
            return '0'