《leetCode》：Implement Queue using Stacks

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 empty operations 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).

思路

用两个Stack来实现队列比较简单，一个用于存储元素，一个用于弹出元素。

实现代码如下：

class MyQueue {
    /* * Stack s1 和s2 用来模拟 * */
    Stack<Integer> s1=new Stack<Integer>();
    Stack<Integer> s2=new Stack<Integer>();
    public void push(int x) {
        s1.push(x);
    }

    public void pop() {
        if(!s2.isEmpty()){
             s2.pop();
        }
        else if(s2.isEmpty()&&!s1.isEmpty()){
            while(!s1.isEmpty()){
                int temp=s1.pop();
                s2.add(temp);
            }
            s2.pop();
        }              
    }

    // Get the front element.
    public int peek() {
        if(!s2.isEmpty()){
            return s2.peek();
        }
        else if(s2.isEmpty()&&!s1.isEmpty()){
            while(!s1.isEmpty()){
                int temp=s1.pop();
                s2.add(temp);
           }
            return s2.peek();
       }
       return -1;//标识队列中没有元素

    }

    // Return whether the queue is empty.
    public boolean empty() {
        return s1.isEmpty()&&s2.isEmpty();
    }
}