队列：空间O(1),时间O(1)

import java.util.*;
public class Solution {
    //Map<Character,Integer> map = new LinkedHashMap<>();
    //Insert one char from stringstream
    int[] nums = new int[128];
    Queue<Character> queue = new LinkedList<>();
    public void Insert(char ch)
    {
        if(nums[ch]++ == 0){
            queue.add(ch);
        }
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        while(!queue.isEmpty()){
            if(nums[queue.peek()]==1)
                return queue.peek();
            else queue.remove();
        }
        return '#';
    }
}

LinkedHashMap：空间O(N),时间O(1)

import java.util.*;
public class Solution {
    Map<Character,Integer> map = new LinkedHashMap<>();
    //Insert one char from stringstream
    public void Insert(char ch)
    {
        if(map.containsKey(ch))
            map.put(ch,map.get(ch)+1);
        else
            map.put(ch,1);
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        for(char ch:map.keySet()){
            if(map.get(ch)==1)
                return ch;
        }
        return '#';
    }
}