232. Implement Queue using Stacks

题目：

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).

链接： http://leetcode.com/problems/implement-queue-using-stacks/

2/28/2017

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