Leetcode Implement Stack using Queues

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).

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解题思路：

只要实现对了push函数，后面三个直接调用队列的函数即可。这种方法的原理就是每次把新加入的数插到前头，这样队列保存的顺序和栈的顺序是相反的，它们的取出方式也是反的，那么反反得正，就是我们需要的顺序了。我们需要一个辅助队列tmp，把s的元素也逆着顺序存入tmp中，此时加入新元素x，再把tmp中的元素存回来，这样就是我们要的顺序了，其他三个操作也就直接调用队列的操作即可。

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 Java Code:

import java.util.LinkedList;
import java.util.Queue;

public class ImStackUsingQue {
    class MyStack {
        private Queue<Integer> q = new LinkedList<Integer>();
        // Push element x onto stack.
        public void push(int x) {
            Queue<Integer> temp = new LinkedList<Integer>();
            while(!q.isEmpty()) {
                temp.add(q.peek());
                q.remove();
            }
            q.add(x);
            while(!temp.isEmpty()) {
                q.add(temp.peek());
                temp.remove();
            }
        }

        // Removes the element on top of the stack.
        public void pop() {
            q.remove();
        }

        // Get the top element.
        public int top() {
            return q.peek();
        }

        // Return whether the stack is empty.
        public boolean empty() {
            return q.isEmpty();
        }
    }
}

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Reference:

1. http://www.cnblogs.com/grandyang/p/4568796.html