title: 剑指 Offer 09. 用两个栈实现队列
tags:

• 栈
• 队列
  categories:
• 算法
• 剑指 Offer

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​

算法

(辅助栈) $O(n)$

一个主栈 $stk$ 用于存储队列元素，一个辅助栈 $cache$ 用于暂存中间状态

1. appendTail(val)：直接将 $val$ 压入到 $stk$ 中
2. deleteHead()：先将 $stk$ 中的元素弹到 $cache$ 中，此时 $cache$ 的栈顶就是队列的队头，删除即可。

时间复杂度

$O(n)$

空间复杂度

$O(n)$

C++ 代码

class CQueue {
public:
    stack<int> stk, cache;

    CQueue() {
        
    }
    
    void appendTail(int value) {
        stk.push(value);
    }
    
    int deleteHead() {
        if (stk.empty()) return -1;
        while (stk.size()) cache.push(stk.top()), stk.pop();
        int val = cache.top();
        cache.pop();
        while (cache.size()) stk.push(cache.top()), cache.pop();
        return val;
    }
};

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

Java 代码

class CQueue {
    private Stack<Integer> stk;
    private Stack<Integer> cache;

    public CQueue() {
        stk = new Stack<>();
        cache = new Stack<>();
    }

    public void appendTail(int value) {
        stk.push(value);
    }

    public int deleteHead() {
        if (stk.isEmpty()) {
            return -1;
        }
        while (!stk.isEmpty()) {
            cache.push(stk.pop());
        }
        int val = cache.pop();
        while (!cache.isEmpty()) {
            stk.push(cache.pop());
        }
        return val;
    }
}

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

Python 代码

class CQueue:

    def __init__(self):
        self.stk = []
        self.cache = []

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

    def deleteHead(self) -> int:
        if not self.stk:
            return -1
        while self.stk:
            self.cache.append(self.stk.pop())
        val = self.cache.pop()
        while self.cache:
            self.stk.append(self.cache.pop())
        return val

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