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面试题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

解法

Python3

class CQueue:

    def __init__(self):
        self._s1 = []
        self._s2 = []
    
    def _check(self):
        if not self._s2:
            while self._s1:
                self._s2.append(self._s1.pop())

    def appendTail(self, value: int) -> None:
        self._s1.append(value)
        self._check()

    def deleteHead(self) -> int:
        self._check()
        return -1 if not self._s2 else self._s2.pop()


# Your CQueue object will be instantiated and called as such:
# obj = CQueue()
# obj.appendTail(value)
# param_2 = obj.deleteHead()

Java

class CQueue {
    private Stack<Integer> s1 = new Stack<>();
    private Stack<Integer> s2 = new Stack<>();

    public CQueue() {

    }
    
    public void appendTail(int value) {
        s1.push(value);
        check();
    }
    
    public int deleteHead() {
        check();
        return s2.empty() ? -1 : s2.pop();
    }

    private void check() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
    }
}

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

JavaScript

var CQueue = function() {
    this.data = []
    this.helper = []
};
/** 
 * @param {number} value
 * @return {void}
 */
CQueue.prototype.appendTail = function(value) {
    this.data.push(value)
};
/**
 * @return {number}
 */
CQueue.prototype.deleteHead = function() {
    if(this.data.length) {
        while(this.data.length > 1) {
            this.helper.push(this.data.pop())
        }
        let res = this.data.pop()
        while(this.helper.length) {
            this.data.push(this.helper.pop())
        }
        return res
    } else {
        return -1
    }
};

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