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.

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).

Stack类题目。

可采用两个stack。s1为所维护的queue，即存储的数据为先入先出，s2为辅助stack。 每次push数据x时，先将s1中数据移到s2中，将x放到s1中，此时它在s1底部，再将s2中数据移回，则s1仍然维护成为FIFO。

class MyQueue {
    private Stack<Integer> s1 = new Stack<Integer>();
    private Stack<Integer> s2 = new Stack<Integer>();
    private int front;
    
    // Push element x to the back of queue.
    public void push(int x) {
        if(s1.empty()) {
            front = x;
        }
        while(!s1.empty()) {
            s2.push(s1.pop());
        }
        s1.push(x);
        while(!s2.empty()) {
            s1.push(s2.pop());
        }
    }

    // Removes the element from in front of queue.
    public void pop() {
        s1.pop();
        if(!s1.empty()) {
            front = s1.peek();
        }
    }

    // Get the front element.
    public int peek() {
        return front;
    }

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