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).

class Queue {    stack<int> input, output;public:    void push(int x) {        input.push(x);    }    void pop(void) {        peek();        output.pop();    }    int peek(void) {        if (output.empty())            while (input.size())                output.push(input.top()), input.pop();        return output.top();    }    bool empty(void) {        return input.empty() && output.empty();    }};