Lua部分抽样：不放回抽取k个元素

-- 错误：取一个删一个，搬移加重复风险
local out = {}
for _ = 1, k do
    local i = math.random(#pool)
    out[#out + 1] = pool[i]
    table.remove(pool, i)
end

local function sample(list, k)
    for i = #list, #list - k + 1, -1 do
        local j = math.random(i)
        list[i], list[j] = list[j], list[i]
    end
    local out = {}
    for i = #list - k + 1, #list do
        out[#out + 1] = list[i]
    end
    return out
end