Lua双向队列：两端进出的双端结构

-- 错误：头部插入用整表搬移，头操作即卡
table.insert(queue, 1, v)

local dq, head, tail = {}, 1, 0
local function pushFront(v)
    head = head - 1
    dq[head] = v
end
local function pushBack(v)
    tail = tail + 1
    dq[tail] = v
end
local function popFront()
    if head > tail then return nil end
    local v = dq[head]
    dq[head] = nil
    head = head + 1
    return v
end
local function popBack()
    if head > tail then return nil end
    local v = dq[tail]
    dq[tail] = nil
    tail = tail - 1
    return v
end