Pseudocode

Readme.txt

BEGIN

FUNCTION HTMLElements(str)
    DEFINE validTags AS ["b", "i", "em", "div", "p"]
    CREATE an empty stack
    
    FOR each character in str
        IF it is an opening tag THEN
            PUSH it onto the stack
        ELSE IF it is a closing tag THEN
            IF stack is empty THEN
                RETURN the mismatched closing tag
            ELSE
                POP top element from the stack
                IF popped element does not match current closing tag THEN
                    RETURN the mismatched opening tag
                END IFs
       
    
    IF stack is empty THEN
        RETURN "true"
    ELSE
        RETURN the first unmatched opening tag
    
END FUNCTION

END

this is my gist on stuck