function permgen(a, n)
    n = n or #a         -- 默认n为a的大小
    if n <= 1 then      -- 还需要改变吗？
        printResult(a)
    else
        for i=1,n do
            -- 将第一个元素放到数组末尾
            a[n], a[i] = a[i], a[n]
            -- 生成其余元素的排列
            permgen(a, n-1)
            -- 恢复第i个元素
            a[n], a[i] = a[i], a[n]
        end
    end
end

function printResult (a)
    for i=1,#a do
        io.write(a[i], " ")
    end
    io.write("\n")
end

permgen ({1,2,3,4})

2 3 4 1 
3 2 4 1 
3 4 2 1 
4 3 2 1 
2 4 3 1 
4 2 3 1 
4 3 1 2 
3 4 1 2 
3 1 4 2 
1 3 4 2 
4 1 3 2 
1 4 3 2 
2 4 1 3 
4 2 1 3 
4 1 2 3 
1 4 2 3 
2 1 4 3 
1 2 4 3 
2 3 1 4 
3 2 1 4 
3 1 2 4 
1 3 2 4 
2 1 3 4 
1 2 3 4 

function permgen (a, n)
    n - n or #a
    if n <= 1 then
        coroutine.yield(a)
    else
        <as before>

function permutations (a)
    local co = coroutine.create(function () permgen(a) end)
    return function ()  -- 迭代器
        local code, res = coroutine.resume(co)
        return res
    end
end

for p in permutations {"a", "b", "c"} do
    printResult(p)
end
--> b c a
--> c b a
--> c a b
--> b a c
--> a b c

function permutations (a)
    return coroutine.wrap(function () permgen(a) end)
end