双栈实现队列剑指 Offer 09. 用两个栈实现队列

Posted 2021-11-02 7TribeZ

剑指 Offer 09. 用两个栈实现队列

难度简单330

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

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.isEmpty()){
            return (stack2.pop());
        }
        else{
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        return stack2.isEmpty()? -1:stack2.pop();
    }
}

]