1614. 括号的最大嵌套深度

class Solution:
    def maxDepth(self, s: str) -> int:
        maxD = 0
        if not s:
            return maxD
        stack = []
        for char in s:
            if char=='(':
                stack.append(char)
                maxD = max(maxD,len(stack))
            elif char==')' :
                stack.pop()
        return maxD