lua从固定table中随机出不重复的子table

function random_table(t, num)
    for i,v in pairs(t) do
        local r = math.random(#t)
        local temp = t[i]
        t[i] = t[r]
        t[r] = temp
    end
    num = num or #t
    for i = #t,num+1, -1 do
        t[i] = nil