查找一个字符串中最长的字符串，这个思路就是从左向右找，用i向右找，j标识开始统计的字符。max记录最大长度.如：
Input: "pwwkew"
Output: 3
Explanation: 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.

[1]算法如下，复杂度O(n)

import java.util.HashMap;
import java.util.Map;
class Solution {

    public int lengthOfLongestSubstring(String s) {
        int max = 0;
        Map<Character,Integer> map= new HashMap<Character,Integer>();
        for(int i =0,j=0;i < s.length();i++){
            Character cha = s.charAt(i);
            if(map.containsKey(cha)){
                j = Math.max(j,map.get(cha)+1);
            }
            map.put(cha,i);
            max = Math.max(max,i-j + 1);
        }
        return max;
    }
}