LinkedHashMap是可以保存put进去的顺序的

class Solution {
    public char firstNotRepeatingChar(String s) {
        Map<Character,Integer> map=new LinkedHashMap();
        for(int i=0;i<s.length();i++){
            char c=s.charAt(i);
            map.put(c,map.getOrDefault(c,0)+1);
        }
        for(Map.Entry<Character,Integer> entry:map.entrySet()){
            if(entry.getValue()==1){
                return entry.getKey();
            }
        }
        return '#';
    }
}