Java-Stack&Queue-LeetCode

            Implement Queue using Stacks


            class
             MyQueue {  
    Stack<Integer> s1 = new Stack<>();  
    Stack<Integer> s2 = new Stack<>();  
  
    // Push element x to the back of queue.  publicvoid push(int x) {  
        s1.push(x);  
    }  
  
    // Removes the element from in front of queue.  publicvoid pop() {  
        if(!s2.isEmpty()) s2.pop();  
        else {  
            while(!s1.isEmpty()) s2.push(s1.pop());  
            s2.pop();  
        }  
    }  
  
    // Get the front element.  publicint peek() {  
        if(!s2.isEmpty()) return s2.peek();  
        else {  
            while(!s1.isEmpty()) s2.push(s1.pop());  
            return s2.peek();  
        }  
    }  
  
    // Return whether the queue is empty.  publicboolean empty() {  
        return s1.empty() && s2.empty();  
    }  
}  

            Implement Stack using Queues


            class
             MyStack {  
    
            //
             Push element x onto stack.  
    Queue<Integer> q1 = new LinkedList<Integer>();  
    Queue<Integer> q2 = new LinkedList<Integer>();  
      
    publicvoid push(int x) {  
        q1.offer(x);  
    }  
  
    // Removes the element on top of the stack.  publicvoid pop() {  
        while(q1.size()>1) q2.offer(q1.poll());  
        q1.poll();  
        Queue<Integer> q = q1;  
        q1 = q2;  
        q2 = q;  
    }  
  
    // Get the top element.  publicint top() {  
        while(q1.size()>1) q2.offer(q1.poll());  
        int x = q1.poll();  
        q2.offer(x);  
        Queue<Integer> q = q1;  
        q1 = q2;  
        q2 = q;  
        return x;  
    }  
  
    // Return whether the stack is empty.  publicboolean empty() {  
        return q1.isEmpty();  
    }  
}