3.5---用栈实现队列(CC150)

import java.util.Stack;

class MyQueue {
    Stack<Integer> s1 = new Stack();
    Stack<Integer> s2 = new Stack();

    // Push element x to the back of queue.
    public void push(int x) {
        
        s1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        //思路：s2不空的时候可以直接pop
        if(!s2.empty())
        {
            s2.pop();
        }
        else
        {
            while(!s1.empty())
            {
                s2.push(s1.pop());
            }
            s2.pop();
        }
    }

    // Get the front element.
    public int peek() {
           if(!s2.empty())
        {
            return s2.peek();
        }
        else
        {
            while(!s1.empty())
            {
                s2.push(s1.pop());
            }
            return s2.peek();
        }
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}