Implement Queue by Two Stacks & Implement Stack using Queues

Implement Queue by Two Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

 class MyQueue {

     private Stack<Integer> stack1, stack2;

     public MyQueue() {
         // do initialization if necessary
         stack1 = new Stack<Integer>();
         stack2 = new Stack<Integer>();
     }

     // Push element x to the back of queue.
     public void push(int x) {
         stack1.push(x);
     }

     // Removes the element from in front of queue.
     public void pop() {
         if (stack2.isEmpty()) {
             while(!stack1.isEmpty()) {
                 stack2.push(stack1.pop());
             }
         }
         stack2.pop();
     }

     // Get the front element.
     public int peek() {
         if (stack2.isEmpty()) {
             while(!stack1.isEmpty()) {
                 stack2.push(stack1.pop());
             }
         }
         return stack2.peek();
     }

     // Return whether the queue is empty.
     public boolean empty() {
         return stack1.isEmpty() && stack2.isEmpty();
     }
 }

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, 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).

分析：

用两个queues，第二个queue永远为空，它只用于临时保存数字而已。

 class MyStack {
     // Push element x onto stack.
     LinkedList<Integer> q1 = new LinkedList<Integer>();
     LinkedList<Integer> q2 = new LinkedList<Integer>();

     public void push(int x) {
         q1.offer(x);
     }

     // Removes the element on top of the stack.
     public void pop() {
        if (!empty()) {
            while(q1.size() >= ) {
                q2.offer(q1.poll());
            }
            q1.poll();

            while(q2.size() >= ) {
                q1.offer(q2.poll());
            }
        }
     }

     // Get the top element.
     public int top() {
         int value = ;
         if (!empty()) {
            while(q1.size() >= ) {
                if (q1.size() == ) {
                    value = q1.peek();
                }
                q2.offer(q1.poll());
            }

            while(q2.size() >= ) {
                q1.offer(q2.poll());
            }
        }
        return value;
     }

     // Return whether the stack is empty.
     public boolean empty() {
         return q1.size() == ;
     }
 }