3. Longest Substring Without Repeating Characters

原题

Given a string, find the length of the longest substring without repeating characters.

Examples:

Given "abcabcbb", the answer is "abc", which the length is 3.

Given "bbbbb", the answer is "b", with the length of 1.

Given "pwwkew", the answer is "wke", with the length of 3. Note that the answer must be a substring,"pwke" is a subsequence and not a substring.

代码实现

public int LengthOfLongestSubstring(string s)
        {
            //first it must be substring: that's a continuous interval for s
            StringBuilder sb = new StringBuilder();
            int max = 0, curmax=0;
            for (int i = 0; i < s.Length; i++)
            {
                string substr = sb.ToString();
                int ind = substr.IndexOf(s[i].ToString(), StringComparison.Ordinal);
                if(ind==-1)//no containing s[i]
                {
                    sb.Append(s[i]);
                    curmax++;
                }
                else
                {                
                    if (max < curmax)  max = curmax;
                    //This block especially solves the string like that "abcabc..." , or it would exceed time
                    //if (s.Substring(i).IndexOf(substr, StringComparison.Ordinal) != -1)
                    //{
                    // i += substr.Length-1;
                    // continue;
                    //}
                    // i - substr.Length: start index for substr
                    // i - substr.Length + ind: moves ind numbers
                    i = i - substr.Length + ind;
                    sb.Clear();
                    curmax = 0;
                }
            }
            if (max < curmax) max = curmax;
            return max;
        }

**还有一种更简洁高效的算法，如下所示，代码中做了详细文字说明。

        public int LengthOfLongestSubstring(string s)
        {
            int[] arr = new int[256];
            //why is 255? because 255 describes all ASCII char
            //每个符号取值初始化都为-1，表示这个字符还未出现
            for (int i = 0; i <= 255; i++)
            {
                arr[i] = -1;
            }
            int maxLen = 0, start = -1;
            for (int i = 0; i != s.Length; i++)
            {
                int ch2int = Convert.ToInt32(s[i]);

                if (arr[ch2int] > start) //表示这个字符已经出现，也就是重复
                    start = arr[ch2int]; //修改新一轮字符串的起始位置
                arr[ch2int] = i; //不断刷新arr
                maxLen = Math.Max(maxLen, i - start); //i-start：当前字符串的长度
            }
            return maxLen;
        }

题库

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完成题目索引

http://blog.csdn.net/daigualu/article/details/73008186