232. Implement Queue using Stacks

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 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).

解题要点：

运用stack的特性，先进后出，思路为把s1全部倾倒在s2里，把数加到s2顶端，然后再倾倒在s1里。这时，s1里的顺序为倒叙，因此可以实现queue的方法。

class MyQueue {
    Stack<Integer> s1 = null;
    Stack<Integer> s2 = null;
    int size;
    boolean done;
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
        size = 0;
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(s1.size() > 0){
            // 转到s2
            s2.push(s1.pop());
        }
        s2.push(x);

        while(s2.size() > 0){
            s1.push(s2.pop());
        }
        size++;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        size--;
        return s1.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return s1.peek();    
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return size == 0;
    }
}

/**
 * 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();
 */