leetCode(37):Implement Queue using Stacks 分类： leetCode 2015-07-14 08:42 157人阅读 评论(0) 收藏

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).

两个栈，一个作为压入栈，一个作为弹出栈。当弹出栈为空时，把压入栈中的数据依次弹出并压入到弹出栈中。如果两者均为空，说明队列为空。

class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
push_stack.push(x);
}

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

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

// Return whether the queue is empty.
bool empty(void) {
if (pop_stack.empty() && push_stack.empty())
return true;
else
return false;
}

private:
stack<int> pop_stack;
stack<int> push_stack;
};