(easy)LeetCode 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).

   代码如下：

class MyQueue {
    // Push element x to the back of queue.
    Stack<Integer> stack1=new Stack<Integer>();
    Stack<Integer> stack2=new Stack<Integer>();
    public void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        while(stack1.size()>1) stack2.push(stack1.pop());
        stack1.pop();
        while(stack2.size()>0) stack1.push(stack2.pop());
    }

    // Get the front element.
    public int peek() {
        while(stack1.size()>1) stack2.push(stack1.pop());
        int x= stack1.peek();
        while(stack2.size()>0) stack1.push(stack2.pop());
        return x;
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack1.empty();
    }
}

　　运行结果：