leetcode-232-implement-queue-using-stacks

Description

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.
Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

思路0

題意可知是用棧實現隊列，那麼聲明兩個棧，一個是輸入棧一個是輸出棧。要輸入的數據放進到輸入棧中，而輸出棧把最後的進來的數據放進去出棧。

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