225. Implement Stack using Queues
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
, andis 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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.
两种方法:http://www.geeksforgeeks.org/implement-stack-using-queue/
一种是push保持顺序,一种是pop是保持顺序。
实现是保证pop是的顺序。
class MyStack {
private Queue<Integer> q1 = new LinkedList<>();
private Queue<Integer> q2 = new LinkedList<>();
// Push element x onto stack.
public void push(int x) {
q1.offer(x);
}
// Removes the element on top of the stack.
public void pop() {
if (q1.size() == 0) {
return;
}
while (q1.size() > 1) {
q2.offer(q1.poll());
}
q1.poll();
Queue<Integer> tmp = q1;
q1 = q2;
q2 = tmp;
}
// Get the top element.
public int top() {
if (q1.size() == 0) {
return -1;
}
while (q1.size() > 1) {
q2.offer(q1.poll());
}
int res = q1.peek();
q2.offer(q1.poll());
Queue<Integer> tmp = q1;
q1 = q2;
q2 = tmp;
return res;
}
// Return whether the stack is empty.
public boolean empty() {
return q1.isEmpty();
}
}