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).

• class Stack {public:    queue<int> queue1;    queue<int> queue2;    // Push element x onto stack.    void push(int x) {        if(queue1.empty())        {            queue1.push(x);            while(!queue2.empty())            {                int data = queue2.front();                queue2.pop();                queue1.push(data);            }                    }        else{            queue2.push(x);            while(!queue1.empty())            {                int data = queue1.front();                queue1.pop();                queue2.push(data);            }        }    }    // Removes the element on top of the stack.    void pop() {        if(!queue1.empty())            queue1.pop();        if(!queue2.empty())            queue2.pop();            }    // Get the top element.    int top() {        if(!queue1.empty())           return queue1.front();         if(!queue2.empty())           return queue2.front();            }    // Return whether the stack is empty.    bool empty() {        return queue1.empty()&&queue2.empty();    }};