Why gsub with string patterns instead of one regexp?

A String pattern such as gsub("()", "") is matched literally with a plain substring search, while a regexp like /\(\)|\[\]|\{\}/ (or Regexp.union("()", "[]", "{}")) runs the regexp engine at every position. On the 10,000-character nested case the brute force makes about 5,000 passes, and the regexp version is roughly ten times slower. If you do write the regexp, every bracket must be escaped.

Could gsub! end the loop instead of comparing lengths?

Yes: gsub! edits the string in place and returns nil when nothing matched, so loop { break unless s.gsub!(...) } stops on its own. With three patterns you would need all three to return nil, which is clumsier than comparing length. In-place edits also need a string that is not frozen; the argument here is a mutable copy.

What does pop return on an empty array?

nil, with no exception. nil != ")" is true, so a stray closer such as the first character of ")(" makes the method return false without a separate expected.empty? check.

Why are there parentheses around closer = CLOSER[ch]?

An assignment inside an if condition is legal Ruby and returns the assigned value, nil for a closer. Without the parentheses Ruby warns that you may have meant ==; wrapping the assignment tells both Ruby and the reader that it is deliberate.

Is each_char or chars better for the walk?

s.chars builds an array of every character first, while s.each_char yields them one at a time without that array. Both are O(n) and either passes, but each_char saves an allocation of up to 10,000 strings.

Valid Parentheses in Ruby: Pair Removal and Stack Solutions

is_valid.rb

Ruby 4.0

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