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)

Difficulty:Easy

class Queue {public:    stack<int> in;    stack<int> out;    // Push element x to the back of queue.    void push(int x) {        in.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        if(in.empty())            return;        while(!in.empty())        {            out.push(in.top());            in.pop();        }        out.pop();        while(!out.empty())        {            in.push(out.top());            out.pop();        }        return;    }    // Get the front element.    int peek(void) {        int ans;        while(!in.empty())        {            out.push(in.top());            in.pop();        }        ans = out.top();        //out.pop();        while(!out.empty())        {            in.push(out.top());            out.pop();        }        return ans;    }    // Return whether the queue is empty.    bool empty(void) {        if(in.empty())           return true;        return false;            }};