剑指 Offer 09. 用两个栈实现队列

题目描述

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 1：

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

示例 2：

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

提示：

• 1 <= values <= 10000
• 最多会对 appendTail、deleteHead 进行 10000 次调用

链接：https://leetcode-cn.com/problems/yong-liang-ge-zhan-shi-xian-dui-lie-lcof

题解

class CQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;

    public CQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    
    public void appendTail(int value) {
        stack1.push(value);
    }
    
    public int deleteHead() {
        if(!stack2.empty()) {
            return stack2.pop();
        } 
        if(stack1.empty()) {
            return -1;
        }
        while(!stack1.empty()) {
            stack2.push(stack1.pop());
        }
        return stack2.pop();
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */

返回