1.16 Implement Stack using Queues

Description

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).

Method

one stack

Time and Space Complexity

push : o(1) pop/ top : o(n); empty : o(1)

Code

class MyStack {
Queue<Integer> q = new LinkedList<Integer>();

// Push element x onto stack.
public void push(int x) {
       q.add(x); 
}

// Removes the element on top of the stack.
public void pop() {
    if (q.size() < 2){
        q.poll();
    }
    int size = q.size();
    for (int i = 0; i < size - 1; i++){
         q.add(q.poll());
    }
    q.poll();
}

// Get the top element.
public int top() {
   int size = q.size();
    for (int i = 0; i < size - 1; i++){
         q.add(q.poll());
    }
    int res = q.peek();
    q.add(q.poll());
    return res;
}

// Return whether the stack is empty.
public boolean empty() {
       return q.isEmpty();   
}

}