Nearest greater element to the left of every element

Input:  {9, 4, 15, 6, 2, 10} 
Output: {-1, 9, -1, 15, 6, 15} 

Explanation:
The first element has nothing on the left side, so the answer for first is -1.
Second element 4 has 9 on the left which is greater than 4, so the answer is 9. 
Third element 15 has nothing greater on the left side, so the answer is -1. 
Fourth element 6 has 15 as the nearest greater element on the left, so the answer is 15
Similarly, we get values for the fifth and sixth elements.

import java.util.Scanner;

class NearestGreaterToLeft {
    private static int[] findNearestGreaterToLeft(int[] a) {
        int n = a.length;
        int[] NGL = new int[n];

        for(int i = 0; i < n; i++) {
            NGL[i] = -1;
            for(int j = i-1; j >= 0; j--) {
                if(a[j] > a[i]) {
                    NGL[i] = a[j];
                    break;
                }
            }
        }

        return NGL;
    }

    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        int n = keyboard.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i ++) {
            a[i] = keyboard.nextInt();
        }
        keyboard.close();

        int[] NGL = findNearestGreaterToLeft(a);
        for(int i = 0; i < n; i++) {
            System.out.print(NGL[i] + " ");
        }
        System.out.println();

    }
}

import java.util.Scanner;
import java.util.Stack;

class NearestGreaterToLeft {
    private static int[] findNearestGreaterToLeftUsingStack(int[] a) {
        int n = a.length;
        int[] NGL = new int[n];

        Stack<Integer> s = new Stack<>();
        for(int i = 0; i < n; i++) {
            NGL[i] = -1;
            while(!s.empty()) {
                int top = s.peek();
                if(top > a[i]) {
                    NGL[i] = top;
                    break;
                } else {
                    s.pop();
                }
            }
            s.push(a[i]);
        }

        return NGL;
    }

    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        int n = keyboard.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i ++) {
            a[i] = keyboard.nextInt();
        }
        keyboard.close();

        int[] NGL = findNearestGreaterToLeftUsingStack(a);
        for(int i = 0; i < n; i++) {
            System.out.print(NGL[i] + " ");
        }
        System.out.println();

    }
}