Leetcode #225 Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

Difficulty:Easy

class Stack {public:    // Push element x onto stack.    queue<int> in;    queue<int> out;    void push(int x) {        in.push(x);    }    // Removes the element on top of the stack.    void pop() {        int len = in.size();        while(!in.empty())        {            out.push(in.front());            in.pop();        }        int i = 0;        while(i<len-1)        {            in.push(out.front());            out.pop();            i++;        }        out.pop();        return;    }    // Get the top element.    int top() {        return in.back();    }    // Return whether the stack is empty.    bool empty() {        if(in.empty())           return true;        return false;    }};