NextGreater.java

package Stacks;

import java.util.Stack;

public class NextGreater {

    public static int[] nextGreater(int[] arr) {
        int n = arr.length;
        int[] res = new int[n];

        Stack<Integer> st = new Stack<>();
        st.push(arr[n - 1]);
        res[n - 1] = -1;

        for (int i = n - 2; i >= 0; i--) {
            while (!st.isEmpty() && st.peek() <= arr[i])
                st.pop();
            if (st.size() == 0)
                res[i] = -1;
            else
                res[i] = st.peek();
            st.push(arr[i]);
        }
        return res;
    }

    public static void main(String[] args) {
        int[] arr = {1, 5, 3, 2, 1, 6, 3, 4};
        int[] res = nextGreater(arr);

        for (int i : res) {
            System.out.print(i + " ");
        }
    }
}