Balance parens

balance_parens.rb

# Given a string with possibly unbalanced parentheses, return a string with balanced
# parentheses removing a minimal number of parentheses.
# Example: (abc)((),
# Output:  (abc)()

def balance_parens(str)
  stack = []
  str.each_char.with_index do |char, i|
    case char
    when '('
      stack.push([char, i])
    when ')'
      if !stack.empty? && stack.last[0] == '('
        stack.pop
      else
        stack.push([char, i])
      end
    end
  end
  
  # only unbalanced parentheses are left in stack
  stack.reverse_each do |key, position|
    str[position] = ''
  end
  str
end

p balance_parens("(abc)(()")