leetcode-232-Implement-Queue-using-Stacks

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描述

Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue.
pop() – Removes the element from in front of queue.
peek() – Get the front element.
empty() – Return whether the queue is empty.

分析

用堆栈实现队列，与leetcode-225-Implement-Stack-using-Queues类似。这里的思路是用两个队列(s1, s2)来模拟堆栈的操作。这个问题可以抽象为要取一杯子饼干杯底的那块，我们可以将这杯饼干倒入到一个空杯中。push(x)操作，用stack的push即可，empty()操作，当s1,s2都是empty()，队列即是empty()，peek函数的含义是取得队首的元素（即杯底），如果s2中有元素，那么就取出，没有，就执行一次「倒空杯」操作。

解决方案1（C++）

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

    // Removes the element from in front of queue.
    void pop(void) {
        this->peek();
        return s2.pop();
    }

    // Get the front element.
    int peek(void) {
        if(!s2.empty()) return s2.top();
        while(!s1.empty()){
            int temp = s1.top();
            s2.push(temp);
            s1.pop();
        }
        return s2.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return s1.empty() && s2.empty();
    }
private:
    stack<int> s1, s2;
};

解决方案2（Golang）

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import "container/list"

type MyQueue struct {
    Stack1 *list.List
    Stack2 *list.List
}


func Constructor() MyQueue {
    result := MyQueue{}
    result.Stack1 = list.New()
    result.Stack2 = list.New()
    return result
}


func (this *MyQueue) Push(x int)  {
    this.Stack1.PushBack(x)
}


func (this *MyQueue) Pop() int {
    this.Peek()
    result := this.Stack2.Back()
    this.Stack2.Remove(result)
    intResult, _ := result.Value.(int)
    return intResult
}


func (this *MyQueue) Peek() int {
    if this.Stack2.Len() == 0 {
        for this.Stack1.Len() != 0 {
            now := this.Stack1.Back()
            this.Stack1.Remove(now)
            this.Stack2.PushBack(now.Value.(int))
        }
    }
    result := this.Stack2.Back()
    intResult, _ := result.Value.(int)
    return intResult
}


func (this *MyQueue) Empty() bool {
    if this.Stack1.Len() == 0 && this.Stack2.Len() == 0 {
        return true
    }
    return false
}


/**
 * 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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