FTPrep, 30 Substring with Concatenation of All Words

写这到hard之前，据说要先写一个道 简单对照物 来热身，于是5mins 完成了这段断代码，是LC第3题，Longest Substring Without Repeating Characters

public class Solution {    public int lengthOfLongestSubstring(String s) {        int len = s.length();        if(len==0|| len==1) return len;        int back=0;        int maxLen=1;        HashSet<Character> set= new HashSet<>();        for(int front=0; front<len; ++front){            if(!set.add(s.charAt(front))){                maxLen=Math.max(maxLen, front-back);                while(s.charAt(back)!=s.charAt(front)){                    set.remove(s.charAt(back));                    back++;                }                back++;            }        }        return maxLen=Math.max(maxLen, len-1-back+1);    }}