LeetCode[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.

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).

push()就全都push到stack1中，執行pop()時，若stack2非空，直接執行stack2.pop()，若stack2爲空，將stack1中元素全部壓入stack2，再執行stack2.pop()。執行peek()時類似，就相當於top()

class Queue {
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())
		{
			while (!stack1.empty())
			{
				stack2.push(stack1.top());
				stack1.pop();
			}
		}
		if (!stack2.empty())
			stack2.pop();
	}

	// Get the front element.
	int peek(void) {
		if (stack2.empty())
		{
			while (!stack1.empty())
			{
				stack2.push(stack1.top());
				stack1.pop();
			}
		}
		if (!stack2.empty())
			return stack2.top();
		else
			return 0;
	}

	// Return whether the queue is empty.
	bool empty(void) {
		return stack1.empty() && stack2.empty();
	}
private:
	stack<int> stack1;
	stack<int> stack2;
};