Queue (stack)

java.util.Queue Queue interface

• Queue Inherited from Collection, So the queue itself is also a collection
• A queue can hold a set of elements , But the characteristic is that the access elements must follow the principle of first in, first out
• Common implementation classes :java.util.LinkedList

offer Method : Joining operation , The element is appended to the end of the queue
poll Method : Out of line operation , After getting the first element of the queue, the element is deleted from the queue
peek Method : Reference the team leader element , After obtaining, the element is still in the queue

   Queue<String> queue = new LinkedList<>();
    queue.offer("one");
    queue.offer("two");
    queue.offer("three");
    System.out.println(queue);  //[one, two, three]
  
    String str = queue.poll();
    System.out.println(str);  //one
    System.out.println(queue);  // [two, three]
   
    str  =queue.peek();
    System.out.println(str);   //two
    System.out.println(queue);   // [two, three]

The stack structure

• The stack can hold a set of elements , However, the access elements must follow the principle of "first in, last out". Usually, the stack queue structure is used to complete the forward " back off " Use of such functions
• Deque If the double ended queue is queued from the same side, the stack structure is realized , therefore Deque Also known as stack, it provides a classic way to get in and out of the stack :push,pop

push: The stack operation element will be appended to the top of the stack
pop: The stack, , After getting the first element of the stack, the element is deleted from the stack

Deque<String> stack = new LinkedList<>();
    stack.push("one");
    stack.push("two");
    stack.push("three");
    stack.push("four");
    System.out.println(stack);

    String str = stack.pop();
    System.out.println(str);     //[four, three, two, one]
    System.out.println(stack);   //[three, two, one]

Two terminal queue interface :java.util.Deque

• Deque Inherited from Queue. The feature is that both ends of the queue can be queued
• Common implementation classes :java.util.LinkedList

   Deque<String>deque = new LinkedList<>();
    deque.offer("one");
    deque.offer("two");
    deque.offer("three");
    deque.offer("four");
    System.out.println(deque);
  
    deque.offerFirst("five");    // Join the team from the head of the team 
   
    deque.offerLast("six");    // Join the team from the end of the team , And offer Agreement 

    String str = deque.poll();     // Go out from the head of the team 
      
    str = deque.pollFirst();   // Go out from the head of the team 
   
    str = deque.pollLast();  // Leave the team from the end of the team