[LeetCode]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.

Notes:

• 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).

［思路］利用两个堆栈实现队列结构。最容易想到的，push时，push入一个堆栈中。pop时，把堆栈内所有元素弹出到另一个堆栈中（利用两次后入先出实现先入先出），再把元素弹出。特别注意一点，当第二个堆栈不是全空的时候，pop出堆栈顶即可；全空的时候要把第一个堆栈中元素全部弹出到另一个堆栈。不必把第二个堆栈中元素全部弹回第一个堆栈。

class Queue {
private:
    stack<int> stack1;
    stack<int> stack2;
public:
    // Push element x to the back of queue.
    void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if(!stack2.empty()){
            stack2.pop();
        }
        else{
            while(!stack1.empty()){
                int temp = stack1.top();
                stack1.pop();
                stack2.push(temp);
            }
            stack2.pop();
        }
        
    }

    // Get the front element.
    int peek(void) {
         if(!stack2.empty()){
            return stack2.top();
        }
        else{
            while(!stack1.empty()){
                int temp = stack1.top();
                stack1.pop();
                stack2.push(temp);
            }
            return stack2.top();
        }
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return stack1.empty()&&stack2.empty();
    }
};