Implement Queue using Stacks

Leetcode

題意：

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).

解題思路：

使用兩個stack，push時往第一個push，pop時，pop第二個stack，若第二個stack為空，則把第一個stack中所有元素全倒入第二個，再pop。

class MyQueue {
    // Push element x to the back of queue.
    Stack<Integer> stackOne = new Stack<Integer>();
    Stack<Integer> stackTwo = new Stack<Integer>();
    public void push(int x) {
        stackOne.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        if (!stackTwo.isEmpty()) {
            stackTwo.pop();
        }else {
            while (!stackOne.isEmpty()) {
                stackTwo.push(stackOne.pop());
            }
            stackTwo.pop();
        }
    }

    // Get the front element.
    public int peek() {
        if (!stackTwo.isEmpty()) {
            return stackTwo.peek();
        }else {
            while (!stackOne.isEmpty()) {
                stackTwo.push(stackOne.pop());
            }
            return stackTwo.peek();
        }
    }

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