What happens if a closing parenthesis is encountered but the stack is empty during validation?

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When a closing parenthesis is encountered and the stack is empty during validation, it signifies that there are no corresponding opening parentheses to match with. This is a clear indication of invalid syntax when dealing with parentheses. The stack data structure is typically used to track opening parentheses as they are encountered; once a closing parenthesis is found, the program expects to pop the last opening parenthesis from the stack. An empty stack at this point reveals that an unmatched closing parenthesis exists, leading to an invalid state.

In contrast, valid parentheses must always have matching pairs, ensuring that every opening parenthesis has a corresponding closing parenthesis. Therefore, finding a closing parenthesis without a preceding opening one confirms that the sequence of parentheses is not balanced, making it an invalid structure overall.