Make The String Great — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def makeGood(self, s):
        """
        :type s: str
        :rtype: str
        """
        stk = []
        for ch in s:
            counter_ch = ch.upper() if ch.islower() else ch.lower()
            if stk and stk[-1] == counter_ch:
                stk.pop()
            else:
                stk.append(ch)
        return "".join(stk)