查漏补缺：CRC冗余码+双向链表插入+实现利用栈括号匹配

s->next = p->next;
s->prior = p;
p->next = s;
p->next->prior = s;

public boolean isValid(String s){
  Stack<Character> stack = new Stack<Character>();
  for(char c : s.toCharArray()){
    if(c == '(') stack.push(')');
    if(c == '[') stack.push(']');
    if(c == '{') stack.push('}');
    if(stack.isEmpty() || c!=stack.pop()) return false;
  } 
  return stack.isEmpty();
}

// 二、双端链表
class DoubleLinked {
    private Node head, tail;//头结点和尾结点
    private int size;//链表的元素数目
    public DoubleLinked() {
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.next = tail;
        tail.prev = head;
        size = 0;
    }
    // 在头部插入node结点
    public void addFirst(Node x) {
        x.prev = head;
        x.next = head.next;
        head.next.prev = x;
        head.next = x;
        size++;
    }
    // 移除指定结点
    public void remove(Node x) {
        x.prev.next = x.next;
        x.next.prev = x.prev;
        size--;
    }
    // 删除链表的第一个结点，并返回该结点
    public Node removeLast() {
        if(head.next == tail) return null;//返回空
        Node last = tail.prev;
        remove(last);//删除尾结点；
        return last;
    }
    public int size() {
        return size;
    }
}