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).
详见：https://leetcode.com/problems/implement-stack-using-queues/description/
Java实现：
方法一：两个队列实现，始终保持一个队列为空即可
class MyStack {
private Queue<Integer> queue1;
private Queue<Integer> queue2;
/** Initialize your data structure here. */
public MyStack() {
queue1=new LinkedList<Integer>();
queue2=new LinkedList<Integer>();
}
/** Push element x onto stack. */
public void push(int x) {
if(queue1.isEmpty()){
queue1.offer(x);
while(!queue2.isEmpty()){
queue1.offer(queue2.poll());
}
}else if(queue2.isEmpty()){
queue2.offer(x);
while(!queue1.isEmpty()){
queue2.offer(queue1.poll());
}
}
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
if(queue1.isEmpty()){
return queue2.poll();
}else{
return queue1.poll();
}
}
/** Get the top element. */
public int top() {
if(queue1.isEmpty()){
return queue2.peek();
}else{
return queue1.peek();
}
}
/** Returns whether the stack is empty. */
public boolean empty() {
return queue1.isEmpty()&&queue2.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();
*/
方法二：一个队列实现栈
class MyStack {
private Queue<Integer> queue;
/** Initialize your data structure here. */
public MyStack() {
queue=new LinkedList<Integer>();
}
/** Push element x onto stack. */
public void push(int x) {
queue.offer(x);
for(int i=0;i<queue.size()-1;++i){
queue.offer(queue.poll());
}
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return queue.poll();
}
/** Get the top element. */
public int top() {
return queue.peek();
}
/** Returns 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();
*/
C++实现：
方法一：两个队列实现，始终保持一个队列为空即可
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
if(que1.empty())
{
que1.push(x);
while(!que2.empty())
{
que1.push(que2.front());
que2.pop();
}
}
else if(que2.empty())
{
que2.push(x);
while(!que1.empty())
{
que2.push(que1.front());
que1.pop();
}
}
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int val;
if(que1.empty())
{
val=que2.front();
que2.pop();
}
else if(que2.empty())
{
val=que1.front();
que1.pop();
}
return val;
}
/** Get the top element. */
int top() {
if(que1.empty())
{
return que2.front();
}
else if(que2.empty())
{
return que1.front();
}
}
/** Returns whether the stack is empty. */
bool empty() {
return que1.empty()&&que2.empty();
}
private:
queue<int> que1;
queue<int> que2;
};
/**
* 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();
*/
方法二：一个队列实现栈
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
que.push(x);
for(int i=0;i<que.size()-1;++i)
{
que.push(que.front());
que.pop();
}
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int val=que.front();
que.pop();
return val;
}
/** Get the top element. */
int top() {
return que.front();
}
/** Returns whether the stack is empty. */
bool empty() {
return que.empty();
}
private:
queue<int> que;
};
/**
* 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();
*/