三个数能排出几种顺序

把 [1, 2, 3] 排成一排，一共有几种排法？运行下面这段程序：

def perm(a):
    res = []
    path = []
    used = [False] * len(a)
    def dfs():
        if len(path) == len(a):
            res.append(list(path))
            return
        for i in range(len(a)):
            if used[i]:
                continue
            used[i] = True
            path.append(a[i])
            dfs()
            used[i] = False
            path.pop()
    dfs()
    return res

print(len(perm([1, 2, 3])))