225. Implement Stack using Queues (用队列模拟栈)

225. Implement Stack using Queues

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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.

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class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        if(q1.empty())
        {
            q1.push(x);
            while(!q2.empty())
            {
                q1.push(q2.front());
                q2.pop();
            }
        }
        else
        {
            q2.push(x);
            while(!q1.empty())
            {
                q2.push(q1.front());
                q1.pop();
            }
        }
    }

    // Removes the element on top of the stack.
    void pop() {
       if(q1.empty()) q2.pop();
       else q1.pop();
        
    }

    // Get the top element.
    int top() {
         if(q1.empty()) return q2.front();
        else return q1.front();
    }

    // Return whether the stack is empty.
    bool empty() {
        return q1.empty()&&q2.empty();
    }
    
private:
    queue<int>q1,q2;
};