HashSet

• contains
• add

Stack+

• empty peek pop push(add) size

HashMap

• valueOf(value)
• get(key), put(key, value), isEmpty()
• containsKey(key) containsValue(value) remove(key)
• cnt.merge(x, 1, Integer::sum);

Queue

• Queue q = new LinkedList<>();
• PriorityQueue heap = new PriorityQueue();
• offer(*) poll() peek() isEmpty()

ArrayDeque q = new ArrayDeque<>(); //双端队列

Deque q = new ArrayDeque<>();

• peekFirst() peekLast() pollFirst() pollLast()
• offerFirst(E e) offerLast(E e)

String s

• s.charAt(i)
• s.toCharArray()；

Arrays.fill
Arrays.sort

String.valueOf