OJ题-用栈实现队列(力扣)

题目：

思路：用两个栈将要出队列的元素换个顺序，由先进后出换为先进先出

import java.util.ArrayList;
class MyQueue {
          
   
    private ArrayList<Integer> in;//申请入的栈
    private ArrayList<Integer> out;//申请出的栈
    /** Initialize your data structure here. */
    public MyQueue() {
          
   
        in = new ArrayList<Integer>();//在队列的构造方法里对栈进行构造
        out = new ArrayList<Integer>();
    }
     //这里栈用的是顺序表实现的  
    /** Push element x to the back of queue. */
    public void push(int x) {
          
   
        in.add(x);//尾插
    }   
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
          
   
         if (out.isEmpty()) {
          
   //如果出的栈是空的，则把入的栈中的元素导进出的栈
            int size = in.size();//让出的栈的size=入的栈的size
            for (int i = 0; i < size; i++) {
          
   
                int v = in.remove(in.size() - 1);//用倒序的方法把in插入out
                out.add(v);
            }
        }
        return out.remove(out.size() - 1);//返回的就是out里的元素
    }
    
    /** Get the front element. */
    public int peek() {
          
   
        if (out.isEmpty()) {
          
   
            int size = in.size();
            for (int i = 0; i < size; i++) {
          
   
                int v = in.remove(in.size() - 1);
                out.add(v);
            }
        }
        return out.get(out.size() - 1);//获取最后一个元素
    }
    /** Returns whether the queue is empty. */
    public boolean empty() {
          
   
         return in.isEmpty() && out.isEmpty();//两个栈都为空，才能判断为空
    }
}
/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

执行结果：