Using two stacks to implement queues [two first in first out is first in first out]

Preface

The queue is characterized by first in, first out , The stack is characterized by last in, first out , If LIFO and then new stack , That's first in, first out .

One 、 Queues are implemented with two stacks

Two 、 Two in and out to get the queue

package everyday.simple;

import java.util.Stack;

//  Two stacks realize the first in, first out of queues .
public class CQueue {
    
    /* target： Use a LIFO stack   Implement a first in first out queue .  The way in and out of the stack and the way in the queue just want to change , If you put the elements in the stack on another stack , Then the order of this stack , Namely   The first element enters the stack 1 The order of . */
    Stack<Integer> in, out;

    public CQueue() {
    
        in = new Stack<>();
        out = new Stack<>();
    }

    public void appendTail(int value) {
    
        in.push(value);
    }

    public int deleteHead() {
    
        // if in return, First of all , Make a good arrangement for the location of conditional visits , Try not to nest if and if else; second , Combine blocks of code that do the same thing under different conditions .

        //  Both stacks are empty 
        if (out.isEmpty() && in.isEmpty()) return -1;
        // in There are elements in the stack , hold in All stack elements are taken out
        if (out.isEmpty()) while (!in.isEmpty()) out.push(in.pop());
        // out There are elements in the stack at this time , Just get it out of the stack .
        return out.pop();
    }
}

summary

reference

[1] LeetCode Queues are implemented with two stacks