暴力

class Solution {
 public int lengthOfLongestSubstring(String s) {
        int the_max = 0;
        HashSet<Character> hs = new HashSet<Character>();
        for (int i = 0; i < s.length(); i++) {
            hs.clear();
            int y = i;
            while ( y < s.length() ) {
                if ( hs.contains(s.charAt(y)))
                    break;
                else {
                    hs.add(s.charAt(y));
                    y++;
                }
            }
            the_max = Math.max(hs.size(), the_max);
            hs.clear();
            
        }
        return the_max;
    }
}

第二种解法 双指针做法

class Solution {
 public int lengthOfLongestSubstring(String s) {
            int the_max = 0;
            HashSet<Character> hs = new HashSet<Character>();
            for(int i = 0,j =0 ; i < s.length() ; i++) {
                while(hs.contains(s.charAt(i))) {
                    hs.remove(s.charAt(j));
                    j++;
                }
                hs.add(s.charAt(i));
                the_max = Math.max(i-j+1,the_max);
            }
            hs.clear();
            return the_max;
        }
}