Reject leftover openings
Updated
If the walk ends and the stack still holds openers, they were never closed. That is invalid. Only an empty stack at the end is balanced (plus no earlier mismatch).
What to focus on
"((" leftover two. "(()" leftover one. "()(" leftover one. Matching is not just equal counts. Leftover means unmatched depth.
Worked examples
two leftover
s = "(("- push push
- end with ['(','(']
Result: invalid
one leftover
s = "(()"- push push pop
- end with ['(']
Result: invalid
clean
s = "(())"- end []
Result: valid
Try it
Write isValid(s). s is a string of brackets. After the walk, if the stack is nonempty the string is invalid even if no closer mismatched. At the end of "((" leftover two (invalid). "(()" leftover one (invalid). "(())" empty (valid).
s is a string of the same bracket type. No closer mismatches mid-walk. Leftover openers at the end are in scope. That is this skill.