LeetCode232 Implement Queue using Stacks Java 题解

题目：

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is emptyoperations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

解题：

用栈实现队列，这个比用队列实现栈要麻烦一些，这里用到两个栈，两种思路，第一种就是在进栈的时候，把栈逆序放在另外一个栈，出的时候直接出另外一个栈就可以了。第二种思路，进栈的时候不做任何处理，出栈的时候把栈逆序放在另外一个栈，出另外一个栈。下面就是两种的代码实现。

在进栈的时候进行处理：

class MyQueue {    // Push element x to the back of queue.Stack<Integer> stack=new Stack<>();Stack<Integer> stack2=new Stack<>();    public void push(int x) {    while(!stack.isEmpty())    {    stack2.push(stack.pop());    }    stack2.push(x);    while(!stack2.isEmpty())    {    stack.push(stack2.pop());    }            }    // Removes the element from in front of queue.    public void pop() {        stack.pop();    }    // Get the front element.    public int peek() {        return stack.peek();    }    // Return whether the queue is empty.    public boolean empty() {        return stack.isEmpty();    }}

在出栈的时候进行处理：

class MyQueue2 {    // Push element x to the back of queue.Stack<Integer> stack=new Stack<>();Stack<Integer> stack2=new Stack<>();    public void push(int x) {    while(!stack2.isEmpty())    stack.push(stack2.pop());    stack.push(x);            }    // Removes the element from in front of queue.    public void pop() {        while(!stack.isEmpty())    stack2.push(stack.pop());        stack2.pop();          }    // Get the front element.    public int peek() {    while(!stack.isEmpty())    stack2.push(stack.pop());        return stack2.peek();          }    // Return whether the queue is empty.    public boolean empty() {    while(!stack2.isEmpty())    stack.push(stack2.pop());        return stack.isEmpty();    }}