[leetcode]Design-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.

• 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 {  
public:  
    // 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(out.empty())  
            trans();  
        out.pop();  
    }  
  
    // Get the front element.  
    int peek(void) {  
        if(out.empty())  
            trans();  
        return out.top();  
    }  
  
    // Return whether the queue is empty.  
    bool empty(void) {  
        return out.empty() && in.empty();  
    }  
private:  
    stack<int> in, out;  
    void trans(){  
        while(!in.empty()){  
            out.push(in.top());  
            in.pop();  
        }  
    }  
};