查漏補缺：CRC備援碼+雙向連結清單插入+實作利用棧括号比對

一、CRC備援碼

二、雙向連結清單的插入

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

三、棧實作括号比對

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();
}      

四、手寫雙向連結清單

// 二、雙端連結清單
class DoubleLinked {
    private Node head, tail;//頭結點和尾結點
    private int size;//連結清單的元素數目

    public DoubleLinked() {
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.next = tail;
        tail.prev = head;
        size = 0;
    }

    // 在頭部插入node結點
    public void addFirst(Node x) {
        x.prev = head;
        x.next = head.next;

        head.next.prev = x;
        head.next = x;
        size++;
    }

    // 移除指定結點
    public void remove(Node x) {
        x.prev.next = x.next;
        x.next.prev = x.prev;
        size--;
    }

    // 删除連結清單的第一個結點，并傳回該結點
    public Node removeLast() {
        if(head.next == tail) return null;//傳回空

        Node last = tail.prev;
        remove(last);//删除尾結點；
        return last;
    }

    public int size() {
        return size;
    }
}