用栈实现队列

人类迷惑行为 x 2

创建两个栈 push 和 pop , 入队数据只进入 push , 出队只从 pop 出. 通过将 push 栈中的数据倒入 pop 栈中实现队列功能, 倒数据的过程中需满足两个条件:

1. 从push 栈倒入 pop 栈之前, pop 栈必须为空
2. push 栈中的数据一次性倒完

import java.util.Stack;

public class StackQueue {
    private Stack<Integer> pushStack;
    private Stack<Integer> popStack;

    public StackQueue() {
        pushStack = new Stack<>();
        popStack = new Stack<>();
    }

    public void push(int newEle) {
        stackTransfer(popStack, pushStack);
        pushStack.push(newEle);
    }

    public int poll() {
        stackTransfer(pushStack, popStack);
        if (popStack.isEmpty())
            throw new RuntimeException("Queue is empty");
        return popStack.pop();
    }

    public int peek() {
        stackTransfer(pushStack, popStack);
        if (popStack.isEmpty())
            throw new RuntimeException("Queue is empty");
        return popStack.peek();
    }

    public boolean empty() {
        return pushStack.isEmpty() && popStack.isEmpty();
    }


    public void stackTransfer(Stack stack1, Stack stack2) {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
    }
}

例题: LeetCode #232

使用栈实现队列的下列操作：

• push(x) – 将一个元素放入队列的尾部。

• pop() – 从队列首部移除元素。

• peek() – 返回队列首部的元素。

• empty() – 返回队列是否为空。

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）。

Java

class MyQueue {
    private Stack<Integer> pushStack;
    private Stack<Integer> popStack;

    /** Initialize your data structure here. */
    public MyQueue() {
        pushStack = new Stack<>();
        popStack = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        stackTransfer(popStack, pushStack);
        pushStack.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        stackTransfer(pushStack, popStack);
        if (popStack.isEmpty())
            throw new RuntimeException("Queue is empty");
        return popStack.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        stackTransfer(pushStack, popStack);
        if (popStack.isEmpty())
            throw new RuntimeException("Queue is empty");
        return popStack.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return pushStack.isEmpty() && popStack.isEmpty();
    }

    /** Put elements from stack1 into stack2. */
    public void stackTransfer(Stack stack1, Stack stack2) {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
    }
}

Python

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.push_stack = []
        self.pop_stack = []

    def push(self, x):
        """
        Push element x to the back of queue.
        """
        self.stack_transfer(self.pop_stack, self.push_stack)
        self.push_stack.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        """
        self.stack_transfer(self.push_stack, self.pop_stack)
        if len(self.pop_stack) == 0:
            return None
        return self.pop_stack.pop()

    def peek(self):
        """
        Get the front element.
        """
        self.stack_transfer(self.push_stack, self.pop_stack)
        if len(self.pop_stack) == 0:
            return None
        return self.pop_stack[-1]

    def empty(self):
        """
        Returns whether the queue is empty.
        """
        return len(self.push_stack) == 0 and len(self.pop_stack) == 0

    def stack_transfer(self, stack1, stack2):
        """
        Put elements from stack1 into stack2.
        """
        if len(stack2) == 0:
            while len(stack1) != 0:
                stack2.append(stack1.pop())