Lua快排手写：分治基准的划分递归

-- 错误：冒泡两两交换，平方级在大榜上拖垮结算
for i = 1, #a - 1 do
    for j = 1, #a - i do
        if a[j] > a[j + 1] then
            a[j], a[j + 1] = a[j + 1], a[j]
        end
    end
end

local function qsort(a, lo, hi)
    if lo >= hi then return end
    local p, store = a[hi], lo
    for i = lo, hi - 1 do
        if a[i] < p then
            a[store], a[i] = a[i], a[store]
            store = store + 1
        end
    end
    a[store], a[hi] = a[hi], a[store]
    qsort(a, lo, store - 1)
    qsort(a, store + 1, hi)
end