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面试题 03.04. 化栈为队

English Version

题目描述

实现一个MyQueue类，该类用两个栈来实现一个队列。

示例：

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // 返回 1
queue.pop();   // 返回 1
queue.empty(); // 返回 false

说明：

• 你只能使用标准的栈操作 -- 也就是只有 push to top, peek/pop from top, size 和 is empty 操作是合法的。
• 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。
• 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

解法

• 每次压入元素时，放入第 1 个栈中；
• 第 2 个栈不为空时，不能倒入元素；
• 第 2 个栈为空时，必须将第 1 个栈的所有元素按顺序倒入第 2 个栈中。

Python3

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self._s1, self._s2 = [], []


    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self._s1.append(x)


    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        if len(self._s2) == 0:
            while self._s1:
                self._s2.append(self._s1.pop())
        return self._s2.pop()


    def peek(self) -> int:
        """
        Get the front element.
        """
        if len(self._s2) == 0:
            while self._s1:
                self._s2.append(self._s1.pop())
        return self._s2[-1]


    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return len(self._s1) + len(self._s2) == 0



# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

Java

class MyQueue {
    private Stack<Integer> s1;
    private Stack<Integer> s2;

    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
        return s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

Go

type MyQueue struct {
	s1, s2 []int
}

/** Initialize your data structure here. */
func Constructor() MyQueue {
	return MyQueue{
		s1: make([]int, 0),
		s2: make([]int, 0),
	}
}

/** Push element x to the back of queue. */
func (this *MyQueue) Push(x int) {
	this.s1 = append(this.s1, x)
}

/** Removes the element from in front of queue and returns that element. */
func (this *MyQueue) Pop() int {
	if len(this.s2) == 0 {
		this.transfer()
	}
	v := this.s2[len(this.s2)-1]
	this.s2 = this.s2[:len(this.s2)-1]
	return v
}

/** Get the front element. */
func (this *MyQueue) Peek() int {
	if len(this.s2) == 0 {
		this.transfer()
	}
	return this.s2[len(this.s2)-1]
}

/** Returns whether the queue is empty. */
func (this *MyQueue) Empty() bool {
	return len(this.s1) == 0 && len(this.s2) == 0
}

func (this *MyQueue) transfer() {
	for len(this.s1) > 0 {
		this.s2 = append(this.s2, this.s1[len(this.s1)-1])
		this.s1 = this.s1[:len(this.s1)-1]
	}
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Peek();
 * param_4 := obj.Empty();
 */

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