Implement Stack using Queues
題意:
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).
解題思路:
一旦插入新值時,要把前面的所有值不斷poll offer到queue的尾端。
class MyStack {
// Push element x onto stack.
Queue<Integer> q = new LinkedList<Integer>();
public void push(int x) {
q.offer(x);
for (int i = 0; i < q.size() - 1; i++) {
q.offer(q.poll());
}
}
// Removes the element on top of the stack.
public void pop() {
q.poll();
}
// Get the top element.
public int top() {
return q.peek();
}
// Return whether the stack is empty.
public boolean empty() {
return q.isEmpty();
}
}