Lua递归转迭代：显式栈消除深递归

-- 错误：六千层递归，栈深直接爆
local function walk(node)
    visit(node)
    for _, c in ipairs(node.children) do
        walk(c)
    end
end

local function walkDeep(root, visit)
    local work = {root}
    while #work > 0 do
        local node = table.remove(work)
        visit(node)
        for i = #node.children, 1, -1 do
            work[#work + 1] = node.children[i]
        end
    end
end