实现

private void Permutation(List<int> num, int leftIndex, List<string> strs)
{
    if (leftIndex < num.Count)
    {
        for (int rightIndex = leftIndex; rightIndex < num.Count; rightIndex++)
        {
            Swap(num, leftIndex, rightIndex);
            Permutation(num, leftIndex + 1, strs);
            Swap(num, rightIndex, leftIndex);
        }
    }
    else
    {
        string s = string.Empty;
        for (int i = 0; i < num.Count; i++)
        {
            s += num[i].ToString();
            if (i < num.Count - 1)
                s += "→";
        }
        strs.Add(s);
    }
}
void Swap(List<int> num, int leftIndex, int rightIndex)
{
    int temp = num[leftIndex];
    num[leftIndex] = num[rightIndex];
    num[rightIndex] = temp;
}

示例

List<int> num = new List<int>() { 1, 2 };
Permutation(num);
num = new List<int>() { 1, 2, 3 };
Permutation(num);

private void Permutation(List<int> num)
{
    List<string> strs = new List<string>();
    Permutation(num, 0, strs);

    string strNum = string.Empty;
    for (int i = 0; i < num.Count; i++)
    {
        strNum += num[i].ToString();
        if (i < num.Count - 1)
            strNum += ",";
    }
    Debug.Log(strNum + " 排列组合共 " + strs.Count + " 组");
    for (int i = 0; i < strs.Count; i++)
        Debug.Log(strs[i] + "\n");
}

 效果