Description
Implement the following operations of a stack using queues.
push(x) – Push element x onto stack.
pop() – Removes the element on top of the stack.
top() – Get the top element.
empty() – Return whether the stack is empty.
Example:
MyStack stack = new MyStack();
stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
Notes:
You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
Code
借助队列来实现栈,实现出栈、入栈、返回栈顶元素和判断栈是否为空几个方法
我们首先看下栈的性质:先进后出
而队列的性质:先进先出
如果只借助一个队列,我们在加入元素时,要做的就是把队列中的元素反转,这样在pop元素时,能够保证先进入的元素后出,满足栈的性质
package com.jie.leetcode;
import java.util.LinkedList;
import java.util.Queue;
/**
* Created by lujie on 2018/10/18.
*/
public class MyStack {
private Queue<Integer> queue = new LinkedList<>();
/** Initialize your data structure here. */
public MyStack() {
}
/** Push element x onto stack. */
public void push(int x) {
queue.add(x);
for(int i = 1;i < queue.size(); i++){
queue.add(queue.poll());
}
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return queue.poll();
}
/** Get the top element. */
public int top() {
return queue.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return queue.isEmpty();
}
}