有没有大佬能看看俺的第二题呀。。。只有36%。。求赐教！ public String NS_String (String s, int k)     {         if(k==0)             return s;         PriorityQueue<Character> dui=new PriorityQueue<>((x,y)->(y-x));//最大堆         dui.add(s.charAt(0));         //用最大堆保存最小的k个字符         for(int i=1;i<s.length();i++)         {             char top=dui.peek();             if(s.charAt(i)<top)             {                 if(dui.contains(s.charAt(i)))                     continue;                 if(dui.size()==k)                 {                     dui.poll();                     dui.add(s.charAt(i));                 }                 else                 {                     dui.add(s.charAt(i));                 }             }         }         StringBuilder t=new StringBuilder();         for(int i=0;i<s.length();i++)         {             if(!dui.contains(s.charAt(i)))                 t.append(s.charAt(i));         }         return t.toString();     }