C# 求字符串中是否包含另一字符串的排列

例如，字符串1"abcdefg"就包含有字符串2"cb"，因为字符串1中有"bc"子字符串，经过排列后可为"cb"。

        static bool IsPermutation(string str1, string str2)
        {
            if (String.IsNullOrEmpty(str1)||String.IsNullOrEmpty(str2))
            {
                throw new Exception("input can't be empty");
            }
            Hashtable ht = new Hashtable();
            Hashtable ht2 = new Hashtable();
            for (int i = 0; i < str1.Length; i++)
            {
                if (ht[str1[i]]==null)
                {
                    ht[str1[i]] = 1;
                }
                else
                {
                    ht[str1[i]] = Int32.Parse(ht[str1[i]].ToString()) + 1;
                }
            }
            for (int i = 0; i < str2.Length; i++)
            {
                if (ht2[str2[i]] == null)
                {
                    ht2[str2[i]] = 1;
                }
                else
                {
                    ht2[str2[i]] = Int32.Parse(ht2[str2[i]].ToString()) + 1;
                }
            }
            for (int i = 0; i < str2.Length; i++)
            {
                if (!ht[str2[i]].Equals(ht2[str2[i]]))
                {
                    return false;
                }
            }
            return true;
        }