经典问题，可以把周边都做了，详见ref  http://blog.csdn.net/linhuanmars/article/details/19949159

public class Solution {    public int lengthOfLongestSubstring(String s) {        if(s==null||s.length()<1) return 0;        int w =0, r = 0, len = 1;        HashSet
     
       hs = new HashSet
      
       ();        while(r