def balanced?(a)
    h = {'{' => '}', '(' => ')', '}' => '', ')' => ''}
    return false unless h[a.shift] == a.pop until a.empty? 
    true
end

看了之前的回复才发现没有考虑 {()()} 的情况，

def balanced?(a)
    h = {'{' => '}', '(' => ')'}
    b = []
    until a.empty?
        e = a.shift
        e == h[b.last] ? b.pop : b << e
    end
    b.empty?
end