leetcode225.用队列实现栈，232.用栈实现队列

这两个数据结构设计类问题在我的文章栈，队列，矩阵相关基础题目及答案中已经有详细的解析了~

用队列实现栈题解：

本题思路如下：
用两个队列来实现栈这种结构，当向自己设计的栈结构push数据的时候，其中一个队列正常进行入队操作。当需要我们pop数据的时候，将有数据的那个队列除去队末的那个数字都enqueue(入队)到另一个队列中，并且让指向两个队列的引用交换。代码如下：

class MyStack {
    private Queue<Integer> dataQueue;
    private Queue<Integer> helpQueue;

    /** Initialize your data structure here. */
    public MyStack() {
        this.dataQueue = new LinkedList<>();
        this.helpQueue = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        dataQueue.add(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(dataQueue.isEmpty()){
            throw new RuntimeException("stack is empty");
        }
        while(dataQueue.size() != 1){
            helpQueue.add(dataQueue.poll());
        }
        int res = dataQueue.poll();
        Queue<Integer> tmp = dataQueue;
        dataQueue = helpQueue;
        helpQueue = tmp;
        return res;
    }
    
    /** Get the top element. */
    public int top() {
        if(dataQueue.isEmpty()){
            throw new RuntimeException("stack is empty");
        }
        while(dataQueue.size() != 1){
            helpQueue.add(dataQueue.poll());
        }
        int res = dataQueue.poll();
        Queue<Integer> tmp = dataQueue;
        dataQueue = helpQueue;
        helpQueue = tmp;
        dataQueue.add(res);
        return res;
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return dataQueue.isEmpty() && helpQueue.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();
 */

代码执行结果如下：

用栈实现队列题解：

使用两个栈来实现一个队列结构，其中一个栈为dataStack,另一个为helpStack。当队列发生进队操作时，向dataStack里push数据，当队列发生出队操作时，如果helpStack为空，就将dataStack依次pop出的数据push进helpStack中，然后返回helpStack pop的数据，如果helpStack不为空，就直接返回helpStack pop的数据，代码如下：

class MyQueue {
    private Stack<Integer> dataStack;
    private Stack<Integer> helpStack;

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

代码执行结果: