Leetcode题目：Implement Queue using Stacks

题目：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).

题目解答：

题目中的要求是使用栈来实现一个队列的功能，具体思路很简单，使用到了两个栈来模拟一个队列的功能。

代码如下：

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        s1.push(x);
        if(s2.empty())
        {
            while(!s1.empty())
            {
                s2.push(s1.top());
                s1.pop();
            }
        }
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if(!s2.empty())
        {
            s2.pop();
        }
        else if(!s1.empty())
        {
            while(!s1.empty())
            {
                s2.push(s1.top());
                s1.pop();
            }
            s2.pop();
        }
    }

    // Get the front element.
    int peek(void) {
        if(!s2.empty())
        {
            return s2.top();
        }
        else if(!s1.empty())
        {
            while(!s1.empty())
            {
                s2.push(s1.top());
                s1.pop();
            }
            return s2.top();
        }
        return -1;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        if( (s1.empty()) && (s2.empty()) )
            return true;
        else
            return false;
    }
private:
    stack<int> s1;
    stack<int> s2;
};