[LeetCode]232. Implement Queue using Stackssil

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).

思路：用两个栈，pop时先将栈底元素之上的元素存进另外一个栈中，弹出栈底元素后将其他元素再放回原来的栈中

当栈为空时，添加第一个元素时将该值保存为peek，用在peek()方法中

class MyQueue {    Stack<Integer> stack1 = new Stack<Integer>();    Stack<Integer> stack2 = new Stack<Integer>();    int peek;    // Push element x to the back of queue.    public void push(int x) {        if (stack1.empty() == true) {            peek = x;        }        stack1.push(x);    }    // Removes the element from in front of queue.    public void pop() {        while( !stack1.empty()) {            stack2.push(stack1.pop());        }        stack2.pop();        if (stack2.empty() == true) peek = 0;        else peek = stack2.peek();         while( !stack2.empty()) {            stack1.push(stack2.pop());        }    }     // Get the front element.    public int peek() {        return peek;    }    // Return whether the queue is empty.    public boolean empty() {        return stack1.empty();    }}