Solution 5: Balanced Brackets

Given a string that contains only the characters ( ) [ and ], determine whether the input string contains no unmatched brackets. Brackets are said to match if every opening bracket has its closing complement subsequent in the string. The string is said to have balanced brackets if all brackets are matched and any subset of brackets enclosed within a matched pair of brackets is also a matched pair of brackets.

Return true if the bracket string is balanced, and false otherwise.

def balanced_brackets(string)
  stack = []
  opening = {
    '{' => '}',
    '[' => ']',
    '(' => ')'
  }
  balanced = true
  for bracket in string.split('')
    if opening.keys.include? bracket
      stack.push bracket
    else
      if opening[stack.last] == bracket
        stack.pop
      else
        balanced = false
        break
      end
    end
  end
  balanced
end