[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();
}
};