[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.
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).

跟前面用队列实现堆原理是差不多的。

class MyQueue {

        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();

        // Push element x to the back of queue.
        public void push(int x) {
            s1.push(x);
        }

        // Removes the element from in front of queue.
        public void pop() {
            while (s1.size() > 1)
                s2.push(s1.pop());
            s1.pop();
            while (s2.size() > 0)
                s1.push(s2.pop());
        }

        // Get the front element.
        public int peek() {
            while (s1.size() > 0)
                s2.push(s1.pop());
            int n = s2.peek();
            while (s2.size() > 0)
                s1.push(s2.pop());
            return n;
        }

        // Return whether the queue is empty.
        public boolean empty() {
            return s1.isEmpty();
        }
    }

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