Passing Stack to a function

Question

I am trying to solve a question on hackerrank.But I keep getting emptyStack exception every time.I think it might be because i am passing the stack to getMax function and it is changing the original stack.
I tried to write this in getMax hoping maybe it won't affect the final stack,but it didn't work.

Stack<Integer> s=st;

Can you point out and explain the mistake I am making.

• The program is running perfectly fine for some test cases.(0,2,27)

• Question Link : https://www.hackerrank.com/challenges/maximum-element/copy-from/243724938

• My solution:

  import java.util.*;
  class Solution
  {
  static void getMax(Stack<Integer> st)
  {
   Stack<Integer> s=st;
   int max=s.peek();
   s.pop();
   while(!s.empty())
   {
       if(s.peek()>max)
       max=s.peek();
       s.pop();       
   }
   System.out.println(max);
   }
   public static void main(String args[])
   {
   Stack<Integer> s=new Stack<Integer>();
   Scanner sc=new Scanner(System.in);
   int n=sc.nextInt();
   while(n-->0)
   {
       int c=sc.nextInt();
       if(c==1)
       s.push(sc.nextInt());
       else if(c==2)
       s.pop();
       else
       getMax(s);
   }        
   } 
  }
  

Answer 1

But I keep getting emptyStack exception every time.

Because when you execute stack.pop, the stack is already empty.

In your getMax method, Stack<Integer> s=st this operation does not copy the stack, which means that st and s refer to the same object, so your subsequent operations will affect the original stack.

Change Stack<Integer> s=st; to Stack<Integer> s= (Stack<Integer>) st.clone();.