一个字符串，返回该字符串中最长的连续不重复子串（字符串只包含小写字母）

1. 题目：一个字符串，返回该字符串中最长的连续不重复子串（字符串只包含小写字母）
2. 先是按着正常思路撸了个n平方复杂度算法，（边遍历，边比对）

C#
static bool IsDuplicate(string source,char target)
        {

            for (int i = 0; i < source.Length - 1; i++)
            {
                if (source[i] == target)
                {
                    return true;
                }
            }
            return false;
        }
        //n平方算法
        public static string GetLongestSubDiffStringNN(string s)
        {
            if (string.IsNullOrEmpty(s)||s==" ")
            {
                Console.WriteLine("Empty string!!");
                return null;
            }

            int maxLength = 0, tempLength = 0;
            string tempStr = "", maxStr = "";
            var chars = s.ToCharArray();

            for (int i = 0; i <= chars.Length - 1; i++)
            {
                //if (IsDuplicate(tempStr, chars[i])
                if (tempStr.Contains(chars[i]))
                    {
                    tempLength = 1;
                    tempStr = s[i].ToString();
                }
                else
                {
                    tempLength++;
                    tempStr += s[i];
                    maxStr = tempLength > maxLength ? tempStr : maxStr;
                    maxLength = tempLength > maxLength ? tempLength : maxLength;
                }
            }
            return maxStr;
        }

但是想想n平方复杂度算法是不可容忍的，降低复杂度的重点在于比对是否存在子对象这段逻辑上，于是想说把已存在的字符存在字典里，读取的时候直接拿就好了。

public static string GetLongestSubDiffStringN(string s)
        {
            if (string.IsNullOrEmpty(s) || s == " ")
            {
                Console.WriteLine("Empty string!!");
                return null;
            }

            int maxLength = 0, tempLength = 0;
            string tempStr = "", maxStr = "";
            //用字典存储已存在字符，省去n次的遍历
            Dictionary<char, int> haveCharDic = new Dictionary<char, int>();
            var chars = s.ToCharArray();

            for (int i = 0; i <= chars.Length - 1; i++)
            {
                if (haveCharDic.ContainsKey(s[i]))
                {
                    tempLength = 1;
                    tempStr = s[i].ToString();
                    // 初始化字典
                    haveCharDic = new Dictionary<char, int>();
                    haveCharDic[s[i]] = 1;
                }
                else
                {
                    tempLength++;
                    tempStr += s[i];
                    haveCharDic[s[i]] = 1;
                    maxStr = tempLength > maxLength ? tempStr : maxStr;
                    maxLength = tempLength > maxLength ? tempLength : maxLength;
                }
            }
            return maxStr;
        }