LeetCode.232 Implement Queue using Stacks

最新推荐文章于 2023-10-09 00:28:50 发布

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本文介绍了一种使用两个栈来实现队列的方法，并提供了一种更易于理解的利用List实现队列的方式。这两种方法都能有效地完成队列的基本操作，如push、pop、peek和检查是否为空。

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题目：

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.

Notes:

You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

分析(2个Stack实现-推荐)：

class MyQueue {
    Stack<Integer> stack;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        stack=new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        stack.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        //使用一个新的stack来承载
        int res=0;
        Stack<Integer> dummy=new Stack<>();
        while(!stack.empty()){
            dummy.push(stack.pop());
        }
        res=dummy.pop();
        //将元素放回
        while(!dummy.empty()){
            stack.push(dummy.pop());
        }
        return res;
    }
    
    /** Get the front element. */
    public int peek() {
        //使用一个新的stack来承载
        int res=0;
        Stack<Integer> dummy=new Stack<>();
        while(!stack.empty()){
            dummy.push(stack.pop());
        }
        res=dummy.peek();
        //将元素放回
        while(!dummy.empty()){
            stack.push(dummy.pop());
        }
        return res;
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack.isEmpty();
    }
}

/**
 * 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();
 */

分析2（借用List实现-易理解）：

class MyQueue {
    Stack<Integer> stack;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack=new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        stack.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        int res=0;
        //借助一个list
        List<Integer> list=new ArrayList<Integer>();
        while(!stack.empty()){
            //一直在最前面插入
            list.add(0,stack.pop());
        }
        res=list.get(0);
        //将剩下的元素放回stack
        for(int i=1;i<list.size();i++){
            stack.push(list.get(i));
        }
        return res;
        
    }
    
    /** Get the front element. */
    public int peek() {
        int res=0;
        //借助一个list
        List<Integer> list=new ArrayList<Integer>();
        while(!stack.empty()){
            //一直在最前面插入
            list.add(0,stack.pop());
        }
        res=list.get(0);
        //将剩下的元素放回stack
        for(int i=0;i<list.size();i++){
            stack.push(list.get(i));
        }
        return res;
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack.isEmpty();   
    }
}

/**
 * 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();
 */