Longest valid bracket substring with nesting ranks

Given a string made only of the characters (, ), [, ], { and }, return the length of the longest contiguous substring that is valid.

A string is valid when all of these hold:

1. Every bracket is closed by a bracket of the same kind, and pairs close in the right order (ordinary balanced brackets).
2. The kinds have a rank: curly {} is highest, square [] is in the middle, and round () is lowest. A pair may directly contain pairs of the same or lower rank only. So {[()]}, {{}} and [()()] are valid, but ([]) and [{}] are not.
3. Concatenating valid strings gives a valid string, in any order. For example, ()[]{} is valid.

def longest_valid(s: str) -> int: ...

Return 0 if no valid substring exists.

Constraints: 0 <= len(s) <= 10^5. Aim for O(n) time.

Example 1. s = "){[()]}[]((" returns 8, for the substring "{[()]}[]".

Example 2. s = "({}[()])" returns 6. The string is balanced, but ( may not contain { or [, so the outer pair is not valid. The longest valid substring is "{}[()]".

Reconstructed from the reported question; details may differ from what interviewers ask.