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
, andis 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).
题意:使用栈实现队列的如下操作:
push(x):将x加入队尾
pop():移除队首元素
peek():取队首元素
empty():判断队列是否为空
Notes:
只能使用栈的基本操作:push, pop ,peek, size, isEmpty
思路:使用两个栈。
元素出队时,先将栈stack1中除栈顶元素外的所有元素出栈,存入stack2中;移除栈顶元素(即队尾元素)后,再将stack2中的元素放入stack1中
class MyQueue { Stack<Integer> stack1 = new Stack<>(); Stack<Integer> stack2 = new Stack<>(); private int top; /** Initialize your data structure here. */ public MyQueue() { } /** Push element x to the back of queue. */ public void push(int x) { if(stack1.isEmpty())//stack1.size() == 0 top = x; stack1.push(x); } /** Removes the element from in front of queue and returns that element. */ public int pop() { while(stack1.size() > 1){ top = stack1.pop(); stack2.push(top); } int x = stack1.pop(); while(stack2.size() > 0){ int i = stack2.pop(); stack1.push(i); } return x; } /** Get the front element. */ public int peek() { return top; } /** Returns whether the queue is empty. */ public boolean empty() { return stack1.isEmpty(); } }