Problem
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).
Example
MyStack stack = new MyStack();
stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
Solution
使用两个queue
queue1用于存放数据
queue2用于top和pop操作时暂存数据
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
while(!inner_queue1.empty())
{
inner_queue1.pop();
}
}
/** Push element x onto stack. */
void push(int x) {
inner_queue1.push(x);
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int queue1_size = inner_queue1.size();
while(queue1_size > 1)
{
inner_queue2.push(inner_queue1.front());
inner_queue1.pop();
--queue1_size;
}
int ret = inner_queue1.front();
inner_queue1.pop();
while(!inner_queue2.empty())
{
inner_queue1.push(inner_queue2.front());
inner_queue2.pop();
}
return ret;
}
/** Get the top element. */
int top() {
int queue1_size = inner_queue1.size();
while(queue1_size > 1)
{
inner_queue2.push(inner_queue1.front());
inner_queue1.pop();
--queue1_size;
}
int ret = inner_queue1.front();
inner_queue1.pop();
while(!inner_queue2.empty())
{
inner_queue1.push(inner_queue2.front());
inner_queue2.pop();
}
inner_queue1.push(ret);
return ret;
}
/** Returns whether the stack is empty. */
bool empty() {
return inner_queue1.empty();
}
queue<int> inner_queue1;
queue<int> inner_queue2;
};
/**
* Your MyStack object will be instantiated and called as such:
* MyStack* obj = new MyStack();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->top();
* bool param_4 = obj->empty();
*/
