用 Java 的 LinkedhashMap 实现

LinkedHashMap<Character,Integer> map = new LinkedHashMap<>();
    public void Insert(char ch)
    {
        if(map.containsKey(ch)) {
            map.put(ch,-1);
        } else {
            map.put(ch, 1);
        }
    }
    public char FirstAppearingOnce()
    {
        Iterator<Character> iterator = map.keySet().iterator();
       while (iterator.hasNext()) {
           char cur = iterator.next();
           if(map.get(cur) == 1) {
               return cur;
           }
       }
       return '#';
    }