LeetCode-Implement Queue using Stacks-蒲公英云

LeetCode-Implement Queue using Stacks

Bertha 。 2022-08-04 00:46 238阅读 0赞

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).

这种题可以自己先跑几个样例就会发现是有规律可循的，主要思路是两个stack，一个stack1用于push，另外一个stack2用于pop，关键就在从stack1倒到stack2的过程，还是直接上代码吧，一目了然：

class MyQueue {
    LinkedList<Integer> stack1 = new LinkedList<Integer>();
    LinkedList<Integer> stack2 = new LinkedList<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.
    public void pop() {
       if (stack2.isEmpty()) {
           while (!stack1.isEmpty()) {
               stack2.push(stack1.pop());
           }
       }
       stack2.pop();
    }
    // Get the front element.
    public int peek() {
        if (stack2.isEmpty()) {
           while (!stack1.isEmpty()) {
               stack2.push(stack1.pop());
           }
       }
       return stack2.peek();
    }
    // Return whether the queue is empty.
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}