leetCode(44):Implement Stack using Queues 分类： leetCode 2015-07-19 19:29 50人阅读 评论(0) 收藏

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.

Notes:

You must use only standard operations of a queue -- which means only

push
to back

,

peek/pop from front

,

size

,
and

is empty

operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

两个队列，永远有一个为空，插入时，插入到不为空的队列中，如果都为空则任意；弹出时，把不为空队列中的所有元素（最后一个除外）转移到另一队列当中，并将最后一个不插入，直接删除即可。如果两个队列均为空，则说明栈为空。

class Stack {
public:
// Push element x onto stack.
void push(int x) {//在非空队列中插入
if (aqueue.empty())
bqueue.push(x);
else
aqueue.push(x);
}

// Removes the element on top of the stack.
void pop() {
if (aqueue.empty())
{//转移
while (bqueue.size() != 1)
{
int tmp = bqueue.front();
aqueue.push(tmp);
bqueue.pop();
}
bqueue.pop();
}
else
{
while (aqueue.size() != 1)
{
int tmp = aqueue.front();
bqueue.push(tmp);
aqueue.pop();
}
aqueue.pop();
}
}

// Get the top element.
int top() {
int tmp;
if (aqueue.empty())
{
while (!bqueue.empty())
{
tmp= bqueue.front();
aqueue.push(tmp);
bqueue.pop();
}
}
else
{
while (!aqueue.empty())
{
tmp = aqueue.front();
bqueue.push(tmp);
aqueue.pop();
}
}
return tmp;
}

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