LeetCode-232.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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

使用两个栈，一个输入栈，一个输出栈

输入O(1)，输出O(n)

class MyQueue {//栈 队列 my
    Stack<Integer> stack1;
    Stack<Integer> stack2;

    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<Integer>();
        stack2= new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        stack1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(stack2.isEmpty()){
            while(!stack1.isEmpty()){
                Integer x = stack1.pop();
                stack2.push(x);
            }
        }
        return stack2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if(stack2.isEmpty()){
            while(!stack1.isEmpty()){
                Integer x = stack1.pop();
                stack2.push(x);
            }
        }
        return stack2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack2.isEmpty()&&stack1.isEmpty();
    }
}

相关题

使用队列实现栈 LeetCode225 https://www.cnblogs.com/zhacai/p/10565181.html