local function is_balanced(s)
    if s == "" then
        return true
    end
    local counter_left = 0  -- number of left brackets so far unmatched
    for i = 1, #s do
        local c = s[i]
        if c == "[" then
            counter_left += 1
        elseif counter_left > 0 do
            counter_left -= 1
        else
            return false
        end
    end
    return counter_left == 0
end

print("Checking examples in task description:")
local brackets = {"", "[]", "][", "[][]", "][][", "[[][]]", "[]][[]"}
for brackets as b do
    local res = is_balanced(b)
    local bb = (b != "") ? b : "(empty)"
    print(string.format("%-8s  %s", bb, res ? "OK" : "NOT OK"))
end

print("\nChecking 7 random strings of brackets of length 8:")
math.randomseed(os.time())
for i = 1, 7 do
    local b = ""
    for j = 1, 8 do
        b ..= (math.random(0, 1) == 0) ? "[" : "]"
    end
    local res = is_balanced(b)
    print(string.format("%s  %s", b, res ? "OK" : "NOT OK"))
end