225. Implement Stack using Queues

题目:

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

链接： http://leetcode.com/problems/implement-stack-using-queues/

2/25/2017, Java

主要想法是，pop(), top()基本上是同样的东西用两次，所以麻烦的事情放在push()里来做。

注意Queue interface本身是没有size()函数的，这个size()是来自LinkedList的函数，恰好题目允许。

public class MyStack {
    Queue<Integer> q = new LinkedList<Integer>();

    /** Initialize your data structure here. */
    public MyStack() {
        
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        int size = q.size();
        Integer val;
        q.add(x);
        for (int i = 0; i < size; i++) {
            val = q.poll();
            q.add(val);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return q.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return q.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q.isEmpty();
    }
}

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