[Leetcode]07/09 [Data Structure]Implement Queue using Stacks

/* M1: two stacks push O(n) pop O(1) peek O(1) M2: one stack push O(n) pop O(n) peek O(n) */

class MyQueue {
    /* M1
    private Stack stack, stack2;
    
    // Push element x to the back of queue.
    public void push(int x) {
        while ( !stack.isEmpty() )
        {
            stack2.push( stack1.pop() );
        }
        stack2.push(x);
        while( !stack2.isEmpty() )
        {
            stack.push(  stack2.pop() );
        }
    }

    // Removes the element from in front of queue.
    public void pop() {
        stack.pop();
    }

    // Get the front element.
    public int peek() {
        stack.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack.isEmpty();
    }
    */
    // Last Exceed input : empty if delcare this way: private Stack stack, stack2; 
    private Stack stack = new Stack<>();
    private Stack stack2 = new Stack<>();
    
    public void push ( int x)
    {
        while( !stack2.isEmpty() )
        {
            stack.push( stack2.pop() );
        }
        stack.push(x);
    }
    
    public void pop()
    {
        while ( !stack.isEmpty() )
        {
            stack2.push(stack.pop());
        }
        stack2.pop();
    }
    
    public int peek()
    {
        while ( !stack.isEmpty() )
        {
            stack2.push(stack.pop());
        }
        return stack2.peek();
    }
    
    public boolean empty()
    {
        while(  !stack2.isEmpty() )
        {
            stack.push( stack2.pop() );
        }
        return stack.isEmpty();
    }
    
}