Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

这道题看似简单。可是用遍历方法的话非常麻烦。以下这样的用hashmap处理方法。是在discuss里看到的，非常巧妙。

public class Solution {

    public int lengthOfLongestSubstring(String s) {

       if(s.length()==0) return 0;

       HashMap<Character,Integer> map=new HashMap<Character,Integer>();

       int max=0;

       for(int i=0,j=0;i<s.length();i++){

            if(map.containsKey(s.charAt(i))){

                j = Math.max(j,map.get(s.charAt(i))+1);

            }

            map.put(s.charAt(i),i);

            max = Math.max(max,i-j+1);

       }

       return max;

   }

}