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).

Solution：Two stacks 倒一下

屏幕快照 2017-09-19 下午4.44.28.png

Time Complexity: O(1) amortized Space Complexity: O(N)

Solution1a Code：

class MyQueue {

    Deque<Integer> input = new ArrayDeque();
    Deque<Integer> output = new ArrayDeque();
    
    public void push(int x) {
        input.push(x);
    }

    public int pop() {
        peek();
        return output.pop();
    }

    public int peek() {
        if (output.isEmpty())
            while (!input.isEmpty()) {
                output.push(input.pop());
            }
        return output.peek();
    }

    public boolean empty() {
        return input.isEmpty() && output.isEmpty();
    }
}