Make the string great

We will iterate over the string and push the characters into the stack. If the top of the stack is the same as the current character but in different cases, we will pop the top of the stack. Otherwise, we will push the current character into the stack.

class Solution:
    def makeGood(self, s: str) -> str:
        stack = []
        for c in s:
            if stack and stack[-1] == c.swapcase():
                stack.pop()
            else:
                stack.append(c)
        return ''.join(stack)

Time complexity: O(n)
Space complexity: O(n)