[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).

这道题是用栈实现队列，题目难度为Easy。

队列和栈的数据存储特点分别是FIFO和FILO，所以队列的push和pop操作肯定有一个要访问栈底部，如果队列头部元素存在栈底，在队列pop和peek时都要访问栈底部，相对要复杂些，所以这里将队列头部元素存在栈顶，这样pop和peek操作可以直接借助栈的pop和top函数。队列的push操作要将新元素放在队列尾部，即栈底，而其他元素不变，所以需要再借助一个栈实现对倒。具体代码：

class Queue {    stack<int> data;public:    // Push element x to the back of queue.    void push(int x) {        stack<int> stk;        while(!data.empty()) {            stk.push(data.top());            data.pop();        }        data.push(x);        while(!stk.empty()) {            data.push(stk.top());            stk.pop();        }    }    // Removes the element from in front of queue.    void pop(void) {        data.pop();    }    // Get the front element.    int peek(void) {        return data.top();    }    // Return whether the queue is empty.    bool empty(void) {        return data.empty();    }};