README.md

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

方法一：双栈

我们可以使用两个栈来实现队列，其中一个栈 stk1 用来存储输入的元素，另一个栈 stk2 用来输出元素。

当调用 appendTail() 方法时，我们将元素压入 stk1 中。

当调用 deleteHead() 方法时，如果此时栈 stk2 为空，我们将栈 stk1 中的元素逐个弹出并压入栈 stk2 中，然后弹出栈 stk2 的栈顶元素即可。如果此时栈 stk2 不为空，我们直接弹出栈 stk2 的栈顶元素即可。如果两个栈都为空，说明队列中没有元素，返回 -1。

时间复杂度上，对于 appendTail() 方法，时间复杂度为 $O(1)$；对于 deleteHead() 方法，时间复杂度为 $O(n)$；空间复杂度为 $O(n)$。其中 $n$ 为队列中的元素个数。

Python3

class CQueue:
    def __init__(self):
        self.stk1 = []
        self.stk2 = []

    def appendTail(self, value: int) -> None:
        self.stk1.append(value)

    def deleteHead(self) -> int:
        if not self.stk2:
            while self.stk1:
                self.stk2.append(self.stk1.pop())
        return -1 if not self.stk2 else self.stk2.pop()


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

Java

class CQueue {
    private Deque<Integer> stk1 = new ArrayDeque<>();
    private Deque<Integer> stk2 = new ArrayDeque<>();

    public CQueue() {
    }

    public void appendTail(int value) {
        stk1.push(value);
    }

    public int deleteHead() {
        if (stk2.isEmpty()) {
            while (!stk1.isEmpty()) {
                stk2.push(stk1.pop());
            }
        }
        return stk2.isEmpty() ? -1 : stk2.pop();
    }
}

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

C++

class CQueue {
public:
    CQueue() {
    }

    void appendTail(int value) {
        stk1.push(value);
    }

    int deleteHead() {
        if (stk2.empty()) {
            while (!stk1.empty()) {
                stk2.push(stk1.top());
                stk1.pop();
            }
        }
        if (stk2.empty()) {
            return -1;
        }
        int ans = stk2.top();
        stk2.pop();
        return ans;
    }

private:
    stack<int> stk1, stk2;
};

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

Go

type CQueue struct {
	stk1, stk2 []int
}

func Constructor() CQueue {
	return CQueue{[]int{}, []int{}}
}

func (this *CQueue) AppendTail(value int) {
	this.stk1 = append(this.stk1, value)
}

func (this *CQueue) DeleteHead() int {
	if len(this.stk2) == 0 {
		for len(this.stk1) > 0 {
			this.stk2 = append(this.stk2, this.stk1[len(this.stk1)-1])
			this.stk1 = this.stk1[:len(this.stk1)-1]
		}
	}
	if len(this.stk2) == 0 {
		return -1
	}
	ans := this.stk2[len(this.stk2)-1]
	this.stk2 = this.stk2[:len(this.stk2)-1]
	return ans
}

/**
 * Your CQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.AppendTail(value);
 * param_2 := obj.DeleteHead();
 */

JavaScript

var CQueue = function () {
    this.stk1 = [];
    this.stk2 = [];
};

/**
 * @param {number} value
 * @return {void}
 */
CQueue.prototype.appendTail = function (value) {
    this.stk1.push(value);
};

/**
 * @return {number}
 */
CQueue.prototype.deleteHead = function () {
    if (!this.stk2.length) {
        while (this.stk1.length) {
            this.stk2.push(this.stk1.pop());
        }
    }
    return this.stk2.length ? this.stk2.pop() : -1;
};

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

TypeScript

class CQueue {
    private stk1: number[];
    private stk2: number[];

    constructor() {
        this.stk1 = [];
        this.stk2 = [];
    }

    appendTail(value: number): void {
        this.stk1.push(value);
    }

    deleteHead(): number {
        if (this.stk2.length == 0) {
            while (this.stk1.length) {
                this.stk2.push(this.stk1.pop());
            }
        }
        return this.stk2.length == 0 ? -1 : this.stk2.pop();
    }
}

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

Rust

struct CQueue {
    s1: Vec<i32>,
    s2: Vec<i32>
}


/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl CQueue {

    fn new() -> Self {
        CQueue {
            s1: Vec::new(),
            s2: Vec::new(),
        }
    }

    fn append_tail(&mut self, value: i32) {
        self.s1.push(value);
    }

    fn delete_head(&mut self) -> i32 {
        match self.s2.pop() {
            Some(value) => value,
            None => {
                while !self.s1.is_empty() {
                    self.s2.push(self.s1.pop().unwrap());
                }
                self.s2.pop().unwrap_or(-1)
            }
        }
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * let obj = CQueue::new();
 * obj.append_tail(value);
 * let ret_2: i32 = obj.delete_head();
 */

C#

public class CQueue {
    private Stack<int> stk1 = new Stack<int>();
    private Stack<int> stk2 = new Stack<int>();

    public CQueue() {

    }

    public void AppendTail(int value) {
        stk1.Push(value);
    }

    public int DeleteHead() {
        if (stk2.Count == 0) {
            while (stk1.Count != 0) {
                stk2.Push(stk1.Pop());
            }
        }
        return stk2.Count == 0 ? -1 : stk2.Pop();
    }
}

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

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