232. Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

题目解析：

• 使用两个栈来实现队列

代码：

class MyQueue {
    Stack<Integer> s1;
    Stack<Integer> s2;
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }
    
    public void push(int x) {
        s1.push(x);
    }
    
    public int pop() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        } 
        return s2.pop();
    }
    
    public int peek() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        } 
        return s2.peek();
    }
    
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

225. Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

题目解析：

• 使用两个队列可以实现栈

代码：

class MyStack {
    Queue<Integer> q1;
    Queue<Integer> q2;
    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
    }
    
    public void push(int x) {
        q1.add(x);
    }
    
    public int pop() {
        while (q1.size() > 1) {
            q2.add(q1.poll());
        }
        int val = q1.poll();
        Queue<Integer> temp = q1;
        q1 = q2;
        q2 = temp;
        return val;
    }
    
    public int top() {
        while (q1.size() > 1) {
            q2.add(q1.poll());
        }
        return q1.peek();
    }
    
    public boolean empty() {
        return q1.isEmpty();
    }
}