Implement Queue using Stacks

该题算是比较简单的，思路是定义两个栈，一个做为入栈，一个做为出栈。入队列的话直接将元素压入入栈，出队列的话首先判断出栈是否为空，为空则将入栈中的元素依次压入出栈中，再从出栈弹出元素。

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        instack.push(x);
        
    }
    // Removes the element from in front of queue.
    void pop(void) {
        if(!outstack.empty()){
            outstack.pop();
            return;
        }
        if(!instack.empty()){
            while(!instack.empty()){
                outstack.push(instack.top());
                instack.pop();
            }
            outstack.pop();
        }
    }
    // Get the front element.
    int peek(void) {
        if(!outstack.empty()){
            return outstack.top();
        }
        if(!instack.empty()){
            while(!instack.empty()){
                outstack.push(instack.top());
                instack.pop();
            }
            return outstack.top();
        }
    }
    // Return whether the queue is empty.
    bool empty(void) {
        return !instack.empty() || !outstack.empty() ? false : true;
        
    }
private: 

//辅助栈
    stack<int> instack, outstack;
};