Why check expected.isEmpty before removeLast()?

removeLast() on an empty List throws a StateError ("Bad state: No element") instead of returning null. A stray closer such as the ) at the start of ")(" hits exactly that case, so the isEmpty test comes first and || short-circuits past the pop.

What does s[i] return in Dart?

A one-character String holding the UTF-16 code unit at i, not a char (Dart has no character type). That is why the stack is a List and the comparison is plain != on strings. For speed you could use s.codeUnitAt(i) and a List of code units instead.

Why is the closer map static const?

A const map literal is created once at compile time and is immutable, and static puts it on the class rather than on each Solution instance. Its inferred type is Map , so _closerFor[ch] is a String? and the closer != null check promotes it to String.

Can I iterate with for (final ch in s.split(''))?

Yes, s.split('') returns a List of single code units and reads nicely in a for-in loop. It allocates the whole list up front, while indexing s[i] does not; both are O(n) and both pass.

Why does the pair-removal loop use var current instead of reassigning s?

Dart allows assigning to a parameter, but a separate var current = s makes it clear that the input is being rewritten round by round. Dart also accepts the method without a trailing return, because flow analysis knows while (true) without a break never finishes normally.

Valid Parentheses in Dart: Pair Removal and Stack Solutions

is_valid.dart

Dart 3.7

Not run yet

+ 7 hidden

Input

s = "{[()()]}"

Expected

true

Each pair closes inside the one that opened before it, and nothing is left open.

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers