591.py

class Solution:
    def isValid(self, code: str) -> bool:
        tags = []
        i, n = 0, len(code)
        while i < n:
            if code[i] != "<":
                if not tags:
                    return False
                i += 1
                continue
            if i == n - 1:
                return False
            if code[i + 1] == "/":
                j = code.find(">", i)
                if j == -1:
                    return False
                tagname = code[i + 2: j]
                if not tags or tags[-1] != tagname:
                    return False
                tags.pop()
                i = j + 1
                if not tags and i != n:
                    return False
            elif code[i + 1] == "!":
                if not tags:
                    return False
                cdata = code[i + 2: i + 9]
                if cdata != "[CDATA[":
                    return False
                j = code.find("]]>", i)
                if j == -1:
                    return False
                i = j + 1
            else:
                j = code.find(">", i)
                if j == -1:
                    return False
                tagname = code[i + 1: j]
                if not 1 <= len(tagname) <= 9 or not all(ch.isupper() for ch in tagname):
                    return False
                tags.append(tagname)
                i = j + 1

        return not tags