题解 | #字符流中第一个不重复的字符#

使用哈希表和队列，每次输入的字符，哈希表中对应的次数+1，当次数为1时，加入队列；同时将队列首部出现次数大于1的字符全部删除。

import java.util.*;

public class Solution {
    //Insert one char from stringstream
    
    Map<Character,Integer> map = new HashMap<>();
    Queue<Character> q = new LinkedList<>();
    
    public void Insert(char ch)
    {
        map.put(ch, map.getOrDefault(ch, 0) + 1);
        if(map.get(ch) == 1) {
            q.offer(ch);
        }
        while(q.size() > 0 && map.get(q.peek()) > 1) {
            q.poll();
        }
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        if(q.size() == 0) {
            return '#';
        } else {
            return q.peek();
        }
    }
}