Why not use replacingOccurrences(of:with:) for the pair removal?

replacingOccurrences(of:with:) is a Foundation method on NSString, so it needs import Foundation, which adds over a second to every Run on this lane. The standard library has no string replace that is fast at this scale, so the brute force works on Array(s.utf8) and removes pairs in its own loop.

Why popLast() and not removeLast()?

removeLast() traps with a fatal error when the array is empty, which is exactly the state a stray closer like the ) at the start of ")(" finds. popLast() returns Character?, nil when empty, and comparing that optional with ch is legal in Swift, so the empty case simply counts as a mismatch.

Can I index the string with s[i]?

Not with an Int. String is a collection of Characters, which can span several bytes, so it is indexed by String.Index and s[i] with an Int does not compile. Iterate with for ch in s, or convert once with Array(s) or Array(s.utf8) when you need integer positions.

Is a [UInt8] stack faster than [Character]?

Yes, a little: comparing Characters has to respect grapheme clusters, while bytes compare as plain integers. For 10,000 brackets both finish in a few milliseconds, so the Character version is kept for readability. For the byte version, iterate s.utf8 and compare with UInt8(ascii: "(").

Why the underscore in func isValid(_ s: String)?

The _ drops the argument label, so the call is isValid("()") rather than isValid(s: "()"). The harness calls the method without labels, so keep the underscore.

Valid Parentheses in Swift: Pair Removal and Stack Solutions

is_valid.swift

Swift 6.2

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