Use of push() and pop()

1、push What is it? ？（ advance ）

push Just push , Extend to push . This is an instruction of assembly ,（ It may also be found in other languages ）. The meaning is almost the same , Is to put an element on the stack . You can imagine that the stack is a box for CDs , There is a bottom but no cover , First push When you go in, you always have to wait push You can't come out until you come out .

 Into the stack  push()
 Push ： At the top of the stack （ The tail of the array ） Add specified elements , And return the length of the new array .

2、pop What is it? ？（ eject ）

pop and push contrary , It means pop up , Is to pop up an element from the stack , Every time it pops up, it's the top of the stack （ The top of the CD case , That is, the last one put in ） That element of .

 Out of the stack  pop()
 Out of the stack ： Delete the top of the stack （ The tail of the array ） An element of , And return the deleted element .

3. Example

requirement ： Given one only includes ‘(’,‘)’,‘{’,‘}’,‘[’,‘]’ String s , Determines whether the string is valid .

Valid string needs to meet ：

1. Opening parentheses must be closed with closing parentheses of the same type .
2. The left parenthesis must be closed in the correct order

Example 1：

 Input ：s = "()"
 Output ：true

Example 2：

 Input ：s = "()[]{}"
 Output ：true

Example 3：

 Input ：s = "([)]"
 Output ：false

    public static boolean isValid(String s) {
    
        int n = s.length();
        if (n % 2 == 1) {
    
            return false;
        }

        Map<Character, Character> pairs = new HashMap<Character, Character>() {
    {
    
            put(')', '(');
            put(']', '[');
            put('}', '{');
        }};
        Deque<Character> stack = new LinkedList<Character>();
        for (int i = 0; i < n; i++) {
    
            char ch = s.charAt(i);
            if (pairs.containsKey(ch)) {
    
                if (stack.isEmpty() || stack.peek() != pairs.get(ch)) {
    
                    return false;
                }
                stack.pop();
            } else {
    
                stack.push(ch);
            }
        }
        return stack.isEmpty();
    }

4. A simple example 2

public static void main(String[] args) {
    
	LinkedList linked=new LinkedList<>();
	linked.add(“ tearful ”);
	linked.add(“ Ash ”);
	linked.add(“ Hey ”);
	System.out.println(linked.pop());// Remove the element at the top of the stack and return its value 
	// return ： tearful 
	linked.push(“ Pig ”);
	System.out.println(linked);// Push the object to the top of the stack 
	//[ Pig ,  Ash ,  Hey ]
}