description： 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).
use two stack to implement queue is a easy think, just remember that, the queue is FIFO!
public class MyQueue { Stack<Integer> stack1; Stack<Integer> stack2; /** Initialize your data structure here. */ public MyQueue() { stack1 = new Stack<Integer>(); stack2 = new Stack<Integer>(); } PRivate void stack2stack() { while (!stack1.isEmpty()) { stack2.push(stack1.pop()); } } /** 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()) { stack2stack(); } return stack2.pop(); } /** Get the front element. */ public int peek() { if (stack2.isEmpty()) { stack2stack(); } return stack2.peek(); } /** Returns whether the queue is empty. */ public boolean empty() { if (stack2.isEmpty()) { stack2stack(); } return stack2.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(); */新聞熱點
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