Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is validValidEvery opening bracket is closed by the same type of bracket, and brackets are closed in the correct order — the most recently opened bracket must be closed first.. A string passes this check when three things hold at once: each closing bracket matches the type of the bracket it's paired with, brackets close in the right nesting order (innermost first), and no closing bracket ever shows up without a matching opener already waiting for it.