leetCode(37):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).

      两个栈，一个作为压入栈，一个作为弹出栈。当弹出栈为空时，把压入栈中的数据依次弹出并压入到弹出栈中。

假设两者均为空，说明队列为空。

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        push_stack.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if (pop_stack.empty())
		{
			while (!push_stack.empty())
			{
				pop_stack.push(push_stack.top());
				push_stack.pop();
			}
			if (!pop_stack.empty())
				pop_stack.pop();
		}
		else
		{
			pop_stack.pop();
		}
    }

    // Get the front element.
    int peek(void) {
        if (pop_stack.empty())
		{
			while (!push_stack.empty())
			{
				pop_stack.push(push_stack.top());
				push_stack.pop();
			}
			if (!pop_stack.empty())
				return pop_stack.top();
		}
		else
		{
			return pop_stack.top();
		}
		return 0;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        if (pop_stack.empty() && push_stack.empty())
			return true;
		else
			return false;
    }
    
private:
	stack<int> pop_stack;
	stack<int> push_stack;
};