Longest Substring Without Repeating Characters

Problem

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.

public class LengthOfSubstrings {
    //第一种方法 暴力解决
    /*public int lengthOfLongestSubstring(String s) {
        int length = 0;
        int lengthmax = 0;
        String sub = "";
        if (s == null) {
            return 0;
        }
        sub+=s.charAt(0)+"";
        lengthmax = 1;
        length = 1;
        for (int j = 1; j < s.length(); j++) {
            for (int i = j; i < s.length(); i++) {
                if (!sub.contains(s.charAt(i) + "")) {
                    length++;
                    sub += s.charAt(i);
                }
                if (lengthmax <= length) {
                    lengthmax = length;
                }
            }
        }
        return lengthmax;
    }*/
    //第二种方法：两个相同字符之间的字符串最短
    /*
    O(n)
    用下标i,j表示子串
    当j遇到重复的字符时，maxlen和j-i比较,并将exist中i-j之间重置为false,重新新的子串
    否则 置为true  继续遍历
    最后 比较 总长-i,max 排除某几个可能
     */
    public int lengthOfLongestSubstring(String s) {
        int s_length = s.length();
        int j = 0;
        int i=0;//i是子串的第一个位置,j是子串的最后一个位置
        boolean exist[] = new boolean[256];//char类型有256个字符
        int maxlength = 0;
        while(j<s_length){
            if(exist[s.charAt(j)]){
                maxlength = Math.max(maxlength,j-i);
                while(s.charAt(i)!=s.charAt(j)){
                    exist[s.charAt(i)]=false;
                    i++;
                }
                i++;
                j++;
            }else{
                exist[s.charAt(j)]=true;
                j++;
            }
        }
        maxlength = Math.max(maxlength,s_length-i);
        return maxlength;
    }
}