Implement Queue by Two Stacks

题目：

As the title described, you should only use two stacks to implement a queue's actions.

The queue should support push(element), pop() and top() where pop is pop the first(a.k.a front) element in the queue.

Both pop and top methods should return the value of first element.

分析：

思路比较重要，只有当stack2空了的时候才需要dump，否则就一直pop或者peak就行

解法：

public class Queue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;

    public Queue() {
       // do initialization if necessary
       stack1 = new Stack<Integer>();
       stack2 = new Stack<Integer>();
    }

    private void dumpElements () {
        while (!stack1.empty() ) {
            stack2.push(stack1.pop() );
        }
    }

    public void push(int element) {
        // write your code here
        stack1.push(element);
    }

    public int pop() {
        // write your code here
        if (stack2.isEmpty() ) {
            this.dumpElements();
        } 
        return stack2.pop();
    }

    public int top() {
        // write your code here
        if (stack2.empty() ) {
            this.dumpElements();
        } 
        return stack2.peek();
    }
}