Implement Queue using Stacks:用栈实现队列

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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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：双栈即可实现队列，比较简单，不啰嗦了。

class MyQueue {
    
    Stack<Integer> s1;
    Stack<Integer> s2;
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<Integer>();
        s2 = new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(!s2.isEmpty()){
            return s2.pop();
        }else{
            if(s1.isEmpty()){
                throw new RuntimeException();
            }else{
                while(!s1.isEmpty()){
                    s2.push(s1.pop());
                }
                return s2.pop();
            }
        }
    }
    
    /** Get the front element. */
    public int peek() {
        if(!s2.isEmpty()){
            return s2.peek();
        }else{
            if(s1.isEmpty()){
                throw new RuntimeException();
            }else{
                while(!s1.isEmpty()){
                    s2.push(s1.pop());
                }
                return s2.peek();
            }
        }
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty()&&s2.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */