Lua环检测：快慢指针判循环

-- 错误：无防护遍历，遇环死循环卡死主线程
local node = start
while node ~= nil do
    visit(node)
    node = nextOf(node)
end

local function hasCycle(start)
    local slow, fast = start, start
    while true do
        slow = nextOf(slow)
        fast = nextOf(nextOf(fast) or start)
        if fast == nil then
            return false
        end
        if slow == fast then
            return true
        end
    end
end