using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int[] arr = {20, 30, 10, 5, 15};

        PrintPreviousGreaterElementNaive(arr);

        PrintPreviousGreaterElement(arr);

    }

​

    public static void PrintPreviousGreaterElementNaive(int[] arr)

    {

        Stack<int> stack = new Stack<int>();

        stack.Push(arr[0]);

        for (int i = 0; i < arr.Length; i++)

        {

            int j;

            for (j = i - 1; j >= 0; j--)

            {

                if (arr[j] > arr[i])

                {

                    Console.Write(arr[j] + " ");

                    break;

                }

            }

​

            if (j == -1)

                Console.Write(-1 + " ");

        }

        Console.WriteLine();

    }

    public static void PrintPreviousGreaterElement(int[] arr)

    {

        Stack<int> stack = new Stack<int>();

        stack.Push(arr[0]);

        for(int i=0;i<arr.Length;i++){

            while(stack.Count>0 && arr[i]>=stack.Peek()){

                stack.Pop();

            }

            int result=stack.Count==0?-1:stack.Peek();

            Console.Write($"{result} ");

            stack.Push(arr[i]);

        }

    }

}