HashMap

1. value++

Map hash = new HashMap<>();
hash.put(cur,hash.getOrDefault(cur,0)+1);

HashMap<Integer,Integer> map = new HashMap<>();
map.get()
map.getOrDefault(cur,defaultValue)：如果cur不存在，则返回默认值
map.put()
map.putIfAbsemt()：如果不存在就插入
value++
map.put(cur,map.getOrDefault(cur,0)+1);
//遍历
for(Map.Entry<Integer,Integer> entry:map.entrySet()){
     int k = entry.getKey();
     int v = entry.getValue();       
}

String

StringBuilder

StringBuilder sb = new StringBuilder();
追加：append，sb.append(1)
删除：delete(0,3)
删除某个字符：deleteCharAt(sb.length()-1)，删除最后一个字符串

String 和 Int转换

String s = "12312";
int i = Integer.valueOf(s).intValue();
int i = Integer.parseInt(s);

数组

Arrays.sort(arr, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1-o2;//升序
                return o2-o1;//降序
            }
        });

集合

# 集合
ArrayList<Integer> list = new ArrayList<>();
list.get()
list.add()
list.remove()
list.size()
list.isEmpty()

Collections.sort(list, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1-o2;//升序
                return o2-o1;//降序
            }
        });

集合转数组

ArrayList list = new ArrayList();
Integer[] arr  = new Integer[list.size()];
list.toArray(arr);

数组转集合

1.  遍历
2.  asList()，有局限性
list = Arrays.asList(arr)

队列

Stack<Integer> stack = new Stack<>();
stack.peek()
stack.push()/add()
stack.pop()/remove()
stack.empty()
Queue<Integer> queue = new LinkedList<>();
queue.peek()
queue.offer()/add()
queue.poll()/remove()
queue.isEmpty()
Deque<Integer> deque = new LinkedList<>();
deque.peekFirst()/getFirst()
deque.peekLast()/getLast()
deque.offerFirst/addFirst()
deque.offerLast/addLast()
deque.pollFirst()/removeFirst()
deque.pollLast()/removeLast()
PriorityQueue<Integer> pq = new PriorityQueue<>(new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1-o2;//小根堆，默认
                return o2-o1;//大根堆
            }
        });
pq.peek()
pq.offer()/add()
pq.poll()/remove()
pq.isEmpty()

集合

Set<Integer> set = new HashSet<>();
set.add()
set.remove()
set.contains();
set.isEmpty();