剑指 Offer 09. 用两个栈实现队列

题目：

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

示例：

输入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
输出：[null,null,3,-1]

输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

题解

class CQueue {
    Stack<Integer> stackIn;
    Stack<Integer> stackOut;
    public CQueue() {
        stackIn=new Stack<Integer>();
        stackOut=new Stack<Integer>();
    }
    
    public void appendTail(int value) {
        stackIn.add(value);
    }
    
    public int deleteHead() {
        if(!stackOut.isEmpty()){
            return stackOut.pop();
        }
        while(!stackIn.isEmpty()){
            stackOut.add(stackIn.pop());
        }
        if(!stackOut.isEmpty()){
            return stackOut.pop();
        }
        return -1;
    }
}