Next greater element in an array

NextGreater.java

// The idea is to insert the newly arrived element in the sorted manner and 
// pop all element that is smaller then the newly arrived element.
// https://www.youtube.com/watch?v=8P-Z7Oc8x9I
// don't use stack.peek() == null for checking is the stack is empty. It will throw and error. use stack.isEmpty()

import java.util.*;

class great{
    
    public static void main(String args[]) {
      
      int[] a = new int[]{11, 13, 21, 3, 23};

      Stack<Integer> s = new Stack<Integer>();
      s.push(a[0]);

      for (int i = 1; i < a.length; i++) {
         if (s.peek() != null) {
          while (true) {
            if (s.isEmpty() || s.peek() > a[i]) {
              break;
            }
            System.out.println(s.pop() + ":" + a[i]);
          }
        }
        s.push(a[i]);
      }
       while (!s.isEmpty()) {
         System.out.println(s.pop() + ":" + -1);
       }
    }
}