leetcode--Implement Queue using Stacks

题目： Implement Queue using Stacks

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：push即把元素加入栈底，最后一个输出。把栈为123变为x123，借助stack2先存入321，再存入x，再存入123

class MyQueue {
    // Push element x to the back of queue.
    private Stack<Integer> stack = new Stack<Integer>();
    public void push(int x) {
        Stack<Integer> stack2 = new Stack<Integer>();
        while(stack.size()!=0){
            stack2.push(stack.pop());
        }
        stack.push(x);
        while(stack2.size()!=0){
            stack.push(stack2.pop());
            
        }
    }

    // Removes the element from in front of queue.
    public void pop() {
        stack.pop();
    }

    // Get the front element.
    public int peek() {
        return stack.peek();
    }

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

Two：

class MyQueue {

    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();
    
    public void push(int x) {
        input.push(x);
    }

    public void pop() {
        peek();
        output.pop();
    }

    public int peek() {
        if (output.empty())
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
    }

    public boolean empty() {
        return input.empty() && output.empty();
    }
}