LeetCode-Implement Stack Using Queues

Posted 2020-08-09 LiBlog

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).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

 Solution:

O(1)

class MyStack {
    private Queue queue;
    
    // Push element x onto stack.
    public void push(int x) {
        Queue q = new LinkedList();
        q.add(x);
        q.add(queue);
        queue = q;            
    }

    // Removes the element on top of the stack.
    public void pop() {
        queue.remove();
        queue = (Queue) queue.peek();
    }

    // Get the top element.
    public int top() {
        return (int)queue.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue==null;
    }
}