Implement Queue by Two Stacks

 Implement Queue by Two StacksShow result 

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

The queue should support push(element), pop() andtop() 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.

看代码实现

/** * Created by jason on 2016/3/3. */class Queue {    private Stack<Integer> stack1;    private Stack<Integer> stack2;    public Queue() {        //do something        stack1 = new Stack<Integer>();        stack2 = new Stack<Integer>();    }    public void stack1Tostack2() {        while (!stack1.empty()) {            stack2.push(stack1.peek());            stack1.pop();        }    }    public void push(int element) {        stack1.push(element);    }    public int pop() {        if (stack2.empty()) {            this.stack1Tostack2();        }        return stack2.pop();    }    public int top() {        if (stack2.empty()) {            this.stack1Tostack2();        }        return stack2.peek();    }}