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

• 两个栈，一个负责输入，一个负责输出；
• 执行输入时，只放入输入栈中；
• 执行输出时，将输入栈的所有元素依次出栈，放入输出栈中；
• 根据栈的特点，处于输入栈栈底的元素，在输出栈中便是栈顶；
• 只有输出栈中没有元素时才进行倒放，而非每一次。

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();
 */

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()
 */

Go

type CQueue struct {
	stk1 []int
	stk2 []int
}

func Constructor() CQueue {
	return CQueue{stk1: []int{}, stk2: []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[0 : len(this.stk1)-1]
		}
	}
	if len(this.stk2) == 0 {
		return -1
	}
	ans := this.stk2[len(this.stk2)-1]
	this.stk2 = this.stk2[0 : 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();
 */

C++

class CQueue {
private:
    stack<int> s1, s2;

public:
    CQueue() {
    }

    void appendTail(int value) {
        s1.push(value);
    }

    int deleteHead() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
        if (s2.empty()) {
            return -1;
        }
        int head = s2.top();
        s2.pop();
        return head;
    }
};

TypeScript

class CQueue {
    private stack1: number[];
    private stack2: number[];
    constructor() {
        this.stack1 = [];
        this.stack2 = [];
    }

    appendTail(value: number): void {
        this.stack1.push(value);
    }

    move(): void {
        while (this.stack1.length != 0) {
            this.stack2.push(this.stack1.pop());
        }
    }

    deleteHead(): number {
        if (this.stack2.length == 0) {
            this.move();
        }
        return this.stack2.length == 0 ? -1 : this.stack2.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> stack1;
    private Stack<int> stack2;

    public CQueue() {
        stack1 = new Stack<int>();
        stack2 = new Stack<int>();
    }

    public void AppendTail(int value) {
        stack1.Push(value);
    }

    public int DeleteHead() {
        if (stack2.Count == 0) {
            while (stack1.Count != 0) {
                stack2.Push(stack1.Pop());
            }
        }
        return stack2.Count == 0 ? -1 : stack2.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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