https://blog.codingnow.com/2022/06/ajdjacency_list.html

ajdjacency_list.lua

local e = {}
local f = {}

local function put(n, pos)
    if not(f[n]) then
        f[n] = pos
    else
        local fpos = f[n]
        while e[fpos][3] do
            fpos = e[fpos][3]
        end
        if pos - fpos > 1 then
            e[fpos][3] = pos
        end
    end
end
function add(n1, n2, last)
    if last then
        n2, n1 = n1, n2
    end
    local pos = #e + 1
    local edge = { n1, n2 }
    put(n1, pos)
    put(n2, pos)
    table.insert(e, edge)
end
function find_all_edge(n)
    local r = {}
    local fpos = f[n]
    local edge = e[fpos]
    while true do
        table.insert(r, { edge[1], edge[2] })
        if edge[1] == n then
            if edge[2] > n then
                fpos = fpos + 1
            else
                fpos = edge[3]
            end
        elseif edge[1] < n then
            fpos = edge[3]
        else
            break
        end
        edge = e[fpos]
    end
    return r
end

add(1, 2)
add(1, 4, true)
add(2, 3)
add(2, 5, true)
add(3, 6, true)
add(4, 5)
add(4, 7, true)
add(5, 6)
add(5, 8, true)
add(6, 9, true)
add(7, 8, true)
add(8, 9, true)

-- for k, v in ipairs(e) do
--     print(k, table.unpack(v))
-- end

for _, v in pairs(find_all_edge(5)) do
    print(table.unpack(v))
end