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# 225.Implement-Stack-using-Queues

## 题目描述

mplement 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.
Example:
MyStack stack = new MyStack();
stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
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).

## 代码

Approach 1: Using one queue. push is O(n), others are O(1).
class MyStack
{
Queue<Integer> queue;
public MyStack()
{
}
// Push element x onto stack.
public void push(int x)
{
for(int i=0;i<queue.size()-1;i++)
{
}
}
// Removes the element on top of the stack.
public void pop()
{
queue.poll();
}
// Get the top element.
public int top()
{
return queue.peek();
}
// Return whether the stack is empty.
public boolean empty()
{
return queue.isEmpty();
}
}
/**
* 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();
* boolean param_4 = obj.empty();
*/
Approach #2 Using two queue
push - O(1), POP O(n)
private Queue<Integer> q1 = new LinkedList<>();
private Queue<Integer> q2 = new LinkedList<>();
private int top;
// Push element x onto stack.
public void push(int x) {
top = x;
}
public void pop() {
while (q1.size() > 1) {
top = q1.remove();
}
q1.remove();
Queue<Integer> temp = q1;
q1 = q2;
q2 = temp;
}
public void push(int x) {
top = x;
while (!q1.isEmpty()) {