https://www.acmicpc.net/problem/2438

이중for문 사용
import java.util.Scanner;
public class Main{
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
for(int i = 0; i < n; i++){
for(int j = 0; j <= i; j++){
System.out.print("*");
}
System.out.println();
}
sc.close();
}
}