Tag Validator

Tag Validator is a grammar-validation problem. Scan with an index and maintain a stack of open tag names. Opening and closing tag names must follow the stated format, typically one to nine uppercase letters; an opening tag pushes its name, while a closing tag must equal the stack top before popping. When a CDATA section begins, skip through the next exact ]]> delimiter, but allow it only inside an already open tag. Enforce a single root: the input must start with an opening tag, and once the stack becomes empty no further characters are legal. Carefully check delimiters and end indices before slicing. Each character is scanned a bounded number of times, giving O(n) time and O(n) stack space. Explain that ordinary text is valid only while inside an open root. Empty tags, mismatched close order, malformed names, and unclosed CDATA are decisive edge cases.