Leetcode225. 用队列实现栈(java)

题目：

使用队列实现栈的下列操作：

push(x) – 元素 x 入栈

pop() – 移除栈顶元素

top() – 获取栈顶元素

empty() – 返回栈是否为空

注意:

你只能使用队列的基本操作-- 也就是 push to back, peek/pop from front, size, 和 is empty 这些操作是合法的。

你所使用的语言也许不支持队列。 你可以使用 list 或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。

你可以假设所有操作都是有效的（例如, 对一个空的栈不会调用 pop 或者 top 操作）。

代码：

• 解法一

import java.util.LinkedList;
class MyStack {

    /** Initialize your data structure here. */
    LinkedList<Integer> list;
    int size;
    public MyStack() {
        list=new LinkedList<Integer>();
        size=0;
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        list.addFirst(x);
        size++;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(!list.isEmpty()){
            size--;;
            return list.removeFirst();
        }else{
            return -1;
        }
    }
    
    /** Get the top element. */
    public int top() {
        if(list.isEmpty())
            throw new IllegalArgumentException("stack is empty");
        return list.getFirst();  
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return size==0;
    }
}

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

• 解法二

class MyStack {
Queue<Integer> queue=new LinkedList<Integer>();
    /** Initialize your data structure here. */
    public MyStack() {
        
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for(int i=0;i<queue.size()-1;i++){
            queue.add(queue.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return queue.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}

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

• 别人的代码

class MyStack {

Deque<Integer> queue;

/** Initialize your data structure here. */
public MyStack() {
	queue = new LinkedList<Integer>();
}

/** Push element x onto stack. */
public void push(int x) {
	queue.offerFirst(x);
}

/** Removes the element on top of the stack and returns that element. */
public int pop() {
	if (!queue.isEmpty()) {
		return queue.pollFirst();
	}
	return -1;
}

/** Get the top element. */
public int top() {
	if (!queue.isEmpty()) {
		return queue.peekFirst();
	}
	return -1;
}

/** Returns whether the stack is empty. */
public boolean empty() {
	return queue.isEmpty();
}
}

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