Lua集合运算：交集并集差集的表实现

-- 错误：双循环两两比对，规模一大就拖
for _, x in ipairs(a) do
    for _, y in ipairs(b) do
        if x == y then
            mark(x)
        end
    end
end

local function setOps(a, b)
    local inA, both, onlyB = {}, {}, {}
    for _, v in ipairs(a) do inA[v] = true end
    for _, v in ipairs(b) do
        if inA[v] then
            both[#both + 1] = v
            inA[v] = nil
        else
            onlyB[#onlyB + 1] = v
        end
    end
    local onlyA = {}
    for v in pairs(inA) do onlyA[#onlyA + 1] = v end
    return onlyA, both, onlyB
end